Of course, it is all counterfactual, but could Airbnb have been as successful had they not joined YC? Recall that PG told them to do things that do not scale - by taking professional photos of rentals in NY - which may have been critical to their early success.
And although PG was initially skeptical of their idea, he quickly changed his thinking about how big Airbnb could become. Revealed in another interesting trail of emails exchanged between PG and Fred Wilson (who also passed on Airbnb). [2]
[1] "In fact, when we funded Airbnb, we thought it was too crazy. We couldn't believe large numbers of people would want to stay in other people's places"
http://www.paulgraham.com/founders.html
[2] http://www.paulgraham.com/airbnb.html
http://avc.com/2011/03/airbnb/
Edit: spelling