Think of Tribes meets Battlefield 3 with configurable Mechs. That could be VERY cool. Mechs are plodding b/c they are large, but they are still fast. And they can have jumpjets, too.
The issue is one of implementation more than concept. Presumably, mechs would be significantly tougher and more lethal than infantry, so only a few players would actually be able to use mechs per team while everyone else focuses on taking down the enemy mechs. It would be insanely hard to balance. That and inserting giant robots into a fast-paced CTF game with infantry doesn't work very well. Tribes requires steep and hilly terrain in order to accommodate skiing and that kind of terrain doesn't work well with mechs.
While Tribes with mechs sounds cool in theory, I don't see how it could work without sacrificing everything that makes Tribes so great.