Why this special treatment for mana-abilities? What are the design implications if they didn't go on the stack?
I can think of three reasons:
- It would force arbitrary actions to be reversed.
- It would allow spells that haven't been fully cast to be countered.
- There are already plenty of opportunities to react.
It would force arbitrary actions to be reversed
Consider this rule:
601.2 [...] If, at any point during the casting of a spell, a player is unable to comply with any of the steps listed below, the casting of the spell is illegal; the game returns to the moment before that spell started to be cast.
Players can obtain mana in the middle of casting a spell. If mana abilities used the stack, that means players would be able to cast spells in response. If the spell were to become unable to be cast, one would have to rewind all direct and indirect effects of those spells. Someone could have revealed private information, shuffled their library or lost the game as a result. How do you undo those?
Not only that, it would also make it far more likely for a spell to become unable to be cast, something that should never happen.
For example, consider this scenario where responding to mana abilities would be allowed.
- Alice only has a single creature on the 'field.
- Starts casting Altar's Reap.
- She activates a Swamp's ability.
- In response, Nancy casts Beacon of Destruction
- Nancy kills Alice's creature and shuffles her Beacon of Destruction into her library.
- Alice can no longer complete the casting of her spell (because she can't pay the additional cost), so the game rewinds to the start of the casting of the spell. Nancy needs to unshuffle her library to match the game state.
It would allow spells that haven't been fully cast to be countered
Players can obtain mana in the middle of casting a spell. This happens after placing the spell on the stack. A spell is a card on the stack (or a copy of spell), so it could be the legitimate target of a Cancel cast in response to activating mana abilities. That means one could counter a spell that han't been (fully) cast yet. That's just too complicated and messy to allow.
There are already plenty of opportunities to react
Activating mana abilities is not something someone one normally does for its own sake. It's done as part of something else, such as casting a spell. Allowing this to be interrupted would be detrimental to the game flow and would add confusion. There are plenty of other points at which players can respond.
The earlier problems only surface when activating mana abilities when one would not have priority under the current rules. Could the game allow players to respond to the activation of mana abilities performed when a player has priority? Yes, but it would often be hard to distinguish whether an activation occurred before or during casting. It's far simpler to have mana abilities never use the stack rather that having the player try to figure out whether one can respond to a particular activation or not.
- Or activating an ability. I'm only mentioning spells to keep it simple.
- The steps to casting a spell are:
- 601.2a) Place card on stack.
- 601.2b-d) Make choices, including targets.
- 601.2e) Determine total cost.
- 601.2f) Activate mana abilities.
- 601.2g) Pay the previously determined cost.
There is some history behind the decision. Back in the dark ages of MtG rules, before sixth edition, there were a lot of rules that were unintuitive and confusing. With the release of Fifth Edition, a new type of ability was introduced,
Mana Sources: "Mana sources" are a new category of abilities. They include the ability of lands to be tapped for mana and all abilities that provide mana as interrupts. For example, Llanowar Elves's ability is now considered a mana source, but the ability of Ice Cauldron is not. Mana sources may be used whenever desired and may not be interrupted; there is no gap between playing the ability and resolving it. For example, you can't Rust a Mox; only continuous effects can stop a mana source from producing mana.
So, a Mana Source is anything that generated mana. Spells like Dark Ritual from earlier editions can be found with the card type Mana Source. Fifth Edition rules clarified that they may not be interrupted. This was necessary, because before the stack, Interrupts (depreciated, and later reclassified as Instants) were handled in batches
In the beginning, there was the batch. You played a spell, a flurry of "fast effects" were played in response, and then everything (well, everything except interrupts) resolved using the "last in, first out rule," with no chance of playing more spells in the middle. [...]
The timing issues presented by Red Elemental Blast and Blue Elemental Blast (and later, Hydroblast and Pyroblast) were particularly troublesome. Interrupts made sense when they targeted spells, but what happened when they began targeting permanents? Remember, removing the source of a fast effect does not remove the fast effect itself, but what about stopping the fast effect in response to it being played? [...]
Under the current rules, Mark taps his Prodigal Sorcerer in an attempt to deal 1 damage to Randy. In response, Randy plays Red Elemental Blast, targeting the Prodigal Sorcerer. This destroys the Sorcerer, but not before its "ping" effect goes onto the stack. That 1 damage is still dealt to Randy. Now, under the earliest Magic rules, Mark would announce the tapping of his Sorcerer, but Randy could say, "Hold on a minute! I want to interrupt your effect." This would stop the game, target the Sorcerer with Randy's Red Elemental Blast, and prevent the damage from ever being dealt. This created huge amounts of confusion over timing: The active player was supposed to have priority each turn, but interrupts being used this way seemed to contradict the rule. This problem, however, was corrected before interrupts were done away with entirely. The ruling said that if you targeted a permanent with an interrupt, the interrupt was played "as an instant."
So, under the pre-fifth edition rules, you could kill a Llanowar Elves or Interdict a land tapping for mana to prevent a player from generating enough mana to play a spell, and the game would have to backup to the point of the illegal action of casting a spell you couldn't pay the cost for. Sixth Edition rules and the stack did away with all this confusion. There are no longer batches and special timing rules for Interrupts/Instants, they all go on the Stack. Abilities that generate mana don't go on the stack, amd therefore cannot be responded to, removing the necessity to specify on the cards that they cannot target mana abilities.
I think the simplest answer is that mana abilities are generally used while casting a spell to pay a cost. If they had to be put on the stack, then they could never be used while casting a spell because you would be taking another action before a previous action is complete.
601.2. To cast a spell is to take it from where it is (usually the hand), put it on the stack, and pay its costs, so that it will eventually resolve and have its effect. Casting a spell follows the steps listed below, in order. If, at any point during the casting of a spell, a player is unable to comply with any of the steps listed below, the casting of the spell is illegal; the game returns to the moment before that spell started to be cast (see rule 717, "Handling Illegal Actions"). Announcements and payments can't be altered after they've been made.
601.2h Once the steps described in 601.2a-g are completed, the spell becomes cast. Any abilities that trigger when a spell is cast or put onto the stack trigger at this time. If the spell's controller had priority before casting it, he or she gets priority.
If you follow 601.2 to it's conculsion you see that you could not activate the mana ability if it was not treated differently because you don't have priority while you are 'casting' a spell.
Yes you had priority when you started to cast the spell and you will have it again when you are done casting the spell, but while you are 'casting' it, (Paying its cost, choosing its targets etc) there is no passing of priority so no abilities or other spells can be used/cast.
So if it was treated like a normal ability, which you also can not activate while casting a spell, then you could never use them to pay a cost while casting a spell. they would always have to be used before you started paying the cost.
Mana sources could go on the stack the same way as other effects. The main difference would be that you could be interrupted in between preparing the costs to be payed and actually paying the costs for casting a spell. For instance you could use all your mana sources in preparation for casting a spell, but your opponent reacts by using their own mana sources which will resolve before your own, then after the mana hits the opponent's mana pool using it to cast a spell that discards your entire hand. Since your own mana still has not hit your mana pool you are basically inable to do anything. After you have discarded your hand, you will not be able to use the mana that finally enters your mana pool for the spell that is no longer in your hand.
A workaround for this problem would be to collect your mana in small steps instead of all at once. Thus you would be using a single mana source and asking your opponent whether they want to react or not. Then use the second mana source and again ask your opponent, until you have collected enough mana in your mana pool to cast your spell. Asking your opponent for permission for every single mana source gets very tedious, so instead the ability to respond to mana sources was removed.