The problem is that most people play full guard badly. If you want to play guard, you need to play a very high guard (rubber guard being one type), or a butterfly guard. Those are the two that are relatively safe from strikes. Unless you are very flexible, you won't be able to play a really good high guard. I think there may be room for development of an "under" your opponent game in MMA -- x guard, deep half guard. But so far it hasn't really been put in place yet. Somebody will get there I bet! We've recently seen DLR guard used in MMA to great effect by Aoki. You never know how people are going to innovate.