Its funny the Y-Combinator mafia (and I mean that lovingly) can really be such a great tool for these personal services companies like ZeroCater and Exec. I doubt they're forced to participate but its such a great source of initial users. I think for some of these companies this may be a bigger Y-combinator benefit then the mentoring/cash or VC intros.
There is a pay-it-forward mentality -- Justin.tv serves as an early customer for many YC startups.