I think I'm agreeing with Kami in thinking that MMA are more effective in MMA competitions but probably not more effective toward the goals of Aikido.
My understanding is that a true Aikidoka could not even join one of those competitions because it is against our teaching to challenge others or seek out violent encounters. Tohei Sensei tells of one such experience in his book "Ki in Daily Life." (although he ends up taking the people on)
I also find it telling that the Gracie family invented the UFC and they always seem to win it. Home field advantage?