Yes, if that pound of material is in the frame it could easily be almost twice as strong on many axis of force. Typical recreational riders don't fly down the mountain side at 60 mph. At these higher speeds the forces placed on the frame for short intervals is a lot higher than you think compared to flat stage riding and allowing the bikes to go lighter would lessen the safety factor. It's probably not a worthwhile trade off. The 1 pound lighter bike would increase climbing speeds by a small amount but might actually increase rider fatigue in other long rough flat riding sections.