My thoughts on Magic Bounce:
First of all, this ability is amazing. There is absolutely no doubt that CAP 1 would be a fantastic momentum-shifter with an ability as powerful and useful as this one. (And I can't believe people are bringing up things about how it doesn't do enough. "However, it has a problem; it does very little to offensive momentum". Seriously, this ability is amazing against 99% of teams, especially when it doesn't come with Psychic typing.)
This of course brings us to the #1 problem with Magic Bounce, the ability is a Pokemon in itself. I consider using this ability (and to a slightly lesser degree, abilities like Magic Guard or Unaware) ridiculous due to its potency. Between stopping Hazard support, neutralizing Taunt, reflecting Status, and more, this ability is easily one of the best in the game.
For comparison, it has made two Pokemon – Xatu and Espeon, both of which have bad typing, bad defensive stats, and only Espeon has even decent offensive stats – rise from obscurity to relevant Pokemon in the OU metagame (though I'll admit they seem to be less common than they were at first).
Now we want to give this ability to a Pokemon with great offensive and defensive typing, solid attack and decent speed, and the defensive stats to handle many pertinent threats of the metagame? If we do this, CAP 1 won't be the Pokemon that explored Momentum, it will be "the Brokemon that got Magic Bounce". It'd be like if we had given the Pokemon Dragon/Steel typing with very good stats all around. Sure, it would probably be fantastic for gaining momentum, but there is something to be said for skillful execution, ie making this comparable to the DPP Breloom of 5th Gen, not the DPP Salamence (which was certainly a fantastic momentum in its own right).
tl;dr: Sure, Magic Bounce is great. It's also one of the most powerful abilities around. Do we want to approach momentum with an precise and focused approach or the subtlety of a sledgehammer? Not to mention how easily this could be broken...
It would be nice if people addressed this point before continuing on and reposting "Magic Bounce so good, it gives momentum", which seems to be the general flow of this thread (rehashing the same points about what abilities each person likes).