Well, if you can do it whenever you want, then, in effect, training in Acrobatics gives the power "Standing is a minor action for you." This is one of the problems with stunting. The "sweet spot" between "not as good as a power you paid scarce resources for" and "not even worth bothering with" is pretty narrow. To my mind, stunting ought to be very circumstantial -- you need some reason you can stand as a minor action with a successful acrobatics check NOW, but not every other time you're prone.
The problem with a closed playtest is that the playtesters, because of the means by which they're selected, "play the game right" -- but the rules tend to break only when people play it wrong. (I speak here as a programmer who has had to deal with bugs that never show up during testing but appear constantly "in the field".) I'm sure the stunting rules work fine in the WOTC offices and home games, and in the carefully selected playtest groups. Out in the real world...
As a DM, I'd be tempted to use the Kantian Imperative when a player wanted to do a stunt:"Do I want every player to be able to do this in every encounter?" If the stunt is highly unlikely to be repeated because it relies on environmental features (say, you were next to a chain you could use to quickly hoist yourself up to your feet), I'd be more likely to approve it than if it was justified by "I'm just that awesome!".
