Are you performing a large-scale customer study? Or is this just a smaller focus group gig?

The primary rationale for hiring a user research recruiter would be to obtain a valid, representative sample for statistical inference. If the number of participants is small -- too small to be statistically significant -- then you cross this concern off your list.

Presumably, you know who your ideal users will be. After all, they must be your future customers, right? So if you can't track them down now, then future marketing will be pretty hopless. You can probably contact them on your own without an intermediary.

Another reason for hiring a recruiter might be to inject a buffer layer for impartial selection. Otherwise, you and your colleagues may inadvertently bias the selection process toward people who know you or are already too favorable.

My experience in this area is not vast -- although it sometimes comes into play for larger corporate naming initiatives. So I may be missing other arguments in favor of hiring research recruiters. I'd be glad to hear those from other people so that I can learn something too.