here's the thing: there are lots of great principles to learn from all of these arts. aikido, kung fu, TKD, etc... the problem is that they train without resistance, so the effectiveness in a real situation gets lost for the most part. Wristlocks are super effective: Lucas Leite got caught in one at the Pan Am's this year. You could learn a lot about the intricacies of wristlocks by spending some time with an Aikido guy.
People used to say karate absolutely couldn't work. We can see that isn't true now. It's just that the traditional way we teach and train karate doesn't work. And it can't work by itself as an "ultimate style."