WTF Taekwondo for MMA?

It can be OK, but that particular system will generally not give you good punching, punch defense, leg kick defense, or clinch fighting skills. It's certainly better than nothing, though. If it's the only thing you have available, go take it up and learn the basics of striking. It should give you pretty good footwork and fast, somewhat unpredictable kicks. If you can keep the standing fight at a distance, you could really use that skill. Still, I'd supplement it with boxing or Muay Thai at the earliest possible time.