Originally Posted by
Kapusta
Also, the part about real springs having some damping is true and worth noting, though I think in the case of a steel spring (steel bike frame) this is probably negligible for the current discussion.
The problem is that you can't just think of the frame by itself, you have to look at the bike + rider system. When you add a soft doughy* rider to a bike, you introduce new avenues for energy loss.
* Sorry, no offense intended.