All of our frames are going to flex usless you've boxed it or built a cage that attaches to the frame. Most of our rigs would lose quite a lot of suspension travel if we completely eliminated frame flex. The benifit to no flex in the frame is that suspension travel would become more predictable. Your truck would feel a lot more "tight" and predictable. As long as your frame isn't ripping the body mounts out of the cab or developing cracks, it really isn't an issue. If you're building a rig thats going to see a lot of extreme flex (rocks) you might wanna stiffin the frame and what not, but by then you'd want 4link suspension and coilovers anyway.