Well it's like any martial arts really, but I find that as soon as you limit yourself to one art form, you'll leave yourself vulnerable one way or another.
The beauty of MMA is that, well, it mixes parts of everything and strives for as functional of a fighting system as possible. But it might also fail in this regard, because when your trying to learn a bit of everything you might not be as excellent as someone who has focused on the one thing.