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.
I have never hired one. I've always sourced users either via cold email / call or ran ads to a survey via Facebook or Google.
I'm cheap though and rather learn directly for myself.
My company works with user research recruiters regularly. Here are some insights about agencies before you go ahead and employ one:
1. How may user research recruitments have they done earlier. The more experienced they are, the more they understand that user research recruitments are done on psychographics and not on demographics alone (as in Market Research). If the agency is not experienced, you would have to work a lot to get them to deliver acceptable quality of recruitments.
2. User recruitment agencies can be found across the globe. While working with them in different countries, we have found out that agencies from high trust countries (like Singapore) usually found far better participants with negligible screening. However, in low trust countries (like India), agencies will provide participants of 'passable' quality with a lot screening time at your end.