The only reason I can imagine applying to YC and then not going through with the startup if I didn't get in is if when I applied I wasn't totally sure that I and my cofounder were totally on crack when we thought we could start a startup. Acceptance by YC would, in that situation, give a big confidence boost, and validate the idea. I don't think that's a terribly unreasonable premise. In fact, I wouldn't be surprised if there were some currently successful startups who thought they might not do it if they didn't get into YC.