They're not full suspension frames, they don't flex while you're hopping on the rear wheel... you need to give them a big punch. Talking from personal experience here, this is invariably something one can notice while riding. That's for titanium, got no experience with proper aluminum frames. I'm basing that on opinions from quite a few pro riders with whom I had the chance to speak about frame characteristics. You're writing that flex is a bad thing and should be eliminated. What I was writing is that flex is a good thing if you know how to make a frame work properly. Really don't care about the details, how heavy the rider should or shouldn't be. That's down to the engineers who design these things. None the less, it works and that's what matters.