To be able to get advice from the caliber of YC's network as well as be around teams that are trying to do things that others deem "crazy" or "destined to fail" is an enlightening experience you can't just get anywhere. There's so much I don't know. YC is a great way to bridge some of my many ignorances. Certainly not the only way, though.
I'm not suggesting that wannapreneurs should just blindly shoot off a half-baked idea to every incubator and just sit on their asses waiting for someone to dump money in their laps. Are there people like that? Sure. But setting a little time aside from working on your startup to apply to a once-in-a-lifetime experience that can only help your chances of success is, I think, a justifiable use of time.