I think YC by design cannot be the best thing in some ways for all startups: it converges you towards asking for more money rather than making money (it doesn't stop you from making money but the idea of the demo day is to present to VCs), secondly it is biased in the favor of an acquisition type of an exit strategy rather than a long term slow but steady growth company for the most part, and finally going through them vs doing it yourself is like going to a private university versus being self taught.
I am not saying these are good or bad things, I am just pointing them out.
2) To get out of your bubble. Being surrounded by other intelligent individuals all working toward demo day helps you stay focused and keep moving.
3) The experience. Being able to have gone through the YC system wouldn't hurt.
4) The seed money would be a huge help.
5) Access to VC's with the YC brand name behind you. So again, the money.
6) The alumni network. I hear it is extremely supportive even after YC.
Disclaimer: I applied to Summer '11