Honestly, this is what probably will make the Soul Gem a go to tool for many players. It is one thing that the Gem can revive Downed partners, but it is another that it will allow you to turn the 2 on 1 situation back on your opponent and have both characters on the attack.
That said, I'm sure we are going to see a good bit of "Happy Birthdays" too if it is activated at the wrong time, thus leading to situations where a revived partner can get put down yet again, thus making the revive pointless and the infinity storm wasted.
At least Capcom is at least trying to build the hype now (although it goes without saying that their lackluster roster is still going to hurt them at launch).
Edit: As for the stage count... I don't see too much of a problem with it. If anything else, MvC2 only had eight stages too (with one for the final boss and another for the training stage).