At first I agreed and thought this was a dumb question. Then I realized it's actually a very smart question. And the answer, beyond any shadow of a doubt, is that butterfly guard is one of the best guards imaginable for MMA.
But what you have to understand is that the entire difficulty with butterfly guard is not the effectiveness of the position. The butterfly guard position is extremely effective. The problem with the butterfly guard is that it's not an easy position to enter into as the bottom player. A skilled opponent isn't going to let you have butterfly guard with upper body grips. That's the entire difficulty with the position -- actually getting into it.
Another thing to keep in mind is that butterfly guard isn't like closed guard where you can chill out. Butterfly guard is an attacking position. Once you have your grips, you attack immediately and explosively. Alternatively, a lot of "non-BJJ guys" like Robbie Lawler are extremely effective at using the butterfly guard to simply create distance and get to their feet.
And then of course you have a badass like Demian Maia who literally TKO'd his opponent with a butterfly guard sweep. Watch from 3:40 (Again, the sweep is beautiful, but it's the entry into the position which is where the real work came in.)