The compelling reason to apply would be that going through YC can increase your value by more than 1.07 or whatever number represents break-even on the standard deal. Whether that's true of your startup or not I can't say, but I do know that lots of founders go through YC after raising seed rounds because they're answering that question with a yes.
That means nothing though. If you (N=1) apply to Google and get rejected but take a Facebook job, it doesn't mean people shouldn't apply to Google.
It was a question more for us than general public. I think YC is probably the best first check you can get since they invest in companies with nothing more than an idea.