I've got a carbon fiber handlebar on my mountain bike, it's quite flexy. It actually has some damping characteristics, too. Hardly a "stale pretzel." And aluminum is actually quite a flexy material, relatively speaking. If you don't believe me, check out the difference between steel drop bars and aluminum ones. The reason aluminum frames are generally quite stiff is the dimensions of the tubing, not the material itself. Because of aluminum's lack of a fatigue limit, an "overbuilt" frame with large diameter tubing (to keep the frame from flexing much) is necessary for durability. That being said, I'd never have a broken or cracked aluminum frame repaired because most aluminum that's used in bike frames is heat treated in a very controlled environment at the factory (at least in the case of 6061 aluminum), and this would be difficult, or prohibitively expensive, to replicate in a repair situation.