I've often wondered this this isn't part of the "foundations" portion that is mentioned in the description of the YCombinator program. Not only a PR firm as you mentioned but a couple of training sessions in PR AND ethics before launch even happens. A core set of skills and knowledge is more than merely a founder and a great idea these days.
I count at least three YCombinator startups that have stumbled severely with PR not once but multiple times: Wakemate, Dropbox and AirBnB.
I count at least three YCombinator startups that have stumbled severely with PR not once but multiple times: Wakemate, Dropbox and AirBnB.