If Koth of the Hammer's plus one ability ends at end of turn, doesn't it make it useless?

Question

How does Koth of the Hammer's plus one ability work when it ends right after you use it and the Mountain turned into a creature has summoning sickness (part of the rulings on Gatherer)?

Answer 1

I think you've misunderstood the ruling:

If Koth's first ability animates a Mountain that came under your control that turn, it will have "summoning sickness" and be unable to attack. It will also be unable to be tapped to activate an ability with the {T} symbol in its cost, such as the Mountain's mana ability or the ability granted to it by Koth's emblem.

If the mountain has been under your control since before the beginning of this turn, it does not have "summoning sickness". It's only unable to tap and attack if it's a mountain that you played this turn.

Answer 2

Whether it has summoning sickness or not depends on how long you have controlled the mountain. How long it has been a creature doesn't matter whatsoever.

If you have controlled the mountain since the beginning of your most recent turn, it doesn't have summoning sickness. So you're fine if you played the Mountain on a previous turn.

If you haven't controlled the mountain since the beginning of your most recent turn, it has summoning sickness. If it's a creature, it can't attack, and you can't use its activated abilities with the tap symbol ({T}) or the untap symbol ({Q}) in its cost. (This means you wouldn't be able to use its native mana-producing ability.)

302.6. A creature’s activated ability with the tap symbol or the untap symbol in its activation cost can’t be activated unless the creature has been under its controller’s control continuously since his or her most recent turn began. A creature can’t attack unless it has been under its controller’s control continuously since his or her most recent turn began. This rule is informally called the “summoning sickness” rule.

Answer 3

If we set aside the creature aspect, let's not forget that it becomes untapped. The fact that it's still a mountain means you can tap it for mana again. (If you played the mountain this turn, then it would have summoning sickness and thus be uneligible to tap for mana, since it is an activated ability. But the likelihood of having only one mountain in a Koth deck is unlikely, and you'd clearly pick one that didn't have summoning sickness.) Red doesn't get much for acceleration, so that's pretty big. Even Garruk Wildspeaker in green only untapped two forests. Or, if you have the Koth emblem, you can tap it to do one damage again.

Considering the creature aspect, as long as you controlled the mountain from the beginning of your turn, it's not impacted by summoning sickness. Even if it was, if there was a color to give something haste, it would be red.

It has 4 toughness so that it doesn't immediately die to a lightning bolt (which was in standard at the time). Considering how weak red creatures typically are, having a 4/4 mountain out will do wonders for an Overwhelming Stampede in a red/green deck as another consideration for it even if it did have summoning sickness.

Edit: sandbox testing. Circle of Protection: Green

Answer 4

The mountain would NOT have summoning sickness period because it specifically states that it becomes a 4/4 red elemental creature until end of turn. It's still a land. Therefor the mountain can't be a creature before that turn or after that turn. Also seeing as how it is still a land It doesn't have summoning sickness. Lands don't have summoning sickness. So you can still tap it for mana or attack. And if you activated his +1 on an opponents turn you can use it to block or tap to use the mana for an instant.