In the early days, Magic had crazy, subpar mulligan rules: you could shuffle up and draw a new 7 if your hand had no lands or all lands, otherwise you were out of luck. A long time ago the Paris Mulligan rule was introduced and it seems to have stuck. If you don't like your hand, throw it back and draw 6. If you don't like that one, try again drawing 5, and so on.
This seems like a pretty good rule and I didn't question it, but recently I've been playing "Duels of the Planeswalkers" on XBox (incidentally, if anyone else has that and wants to challenge me, let me know!). The mulligan rule there is a little different. You can throw your hand back and draw a new 7 once for free. After that Paris rules are in force: every subsequent time you mulligan your hand size goes down by one.
It seems to me that this "lenient mulligan" rule is really good for the game. Magic is a game with a large element of randomness from the shuffle. Quite often players will draw a 7-card hand that's basically unplayable and have to throw it back, unless they're crazy gamblers with nothing left to lose. If your opponent keeps and you mulligan to 6, you're at a significant disadvantage already. If you have to mulligan to 5 or fewer, you'd better hope somebody up there likes you.
Does anyone know (or have an informed idea) why Magic retains a mulligan rule that allows unbalanced games to happen as often as they do? It just feels like the XBox mulligan would allow players to play their decks "as they were meant to work" more often, and have to battle against long odds much less often. What is the defence for the continued predominance in the Magic world of the Paris mulligan rule?