Idk, I imagine someone in YC without an idea would work just as hard if not harder than, say, someone who already has a product with some traction. I imagine John Doe would wake up every morning thinking "crap, I'm already behind everyone else! I better get moving" (and he'll have to get something together for demo day, which presumably is what YC will get a piece of).

I'm sure this is impossible to generalize, but I wonder if in some circumstances it might even save people a few weeks of time. If a lot of teams change their idea during YC, presumably in some cases it takes them a while to decide (or to be convinced by the partners) to do so. Whereas if they go in not attached to any particular idea, it might be easier for pg & crew to talk them out of their bad ideas so that they reach a good idea more quickly.