This seems somewhat useful. I've definitely found I have a much better signal-to-noise ratio on more focused job boards. The more focused, the better.
Eg when hiring at tiny startups, WorkAtAStartup (or Wellfound, if I'm not currently at a YC company) get a lot more candidates who are actually interested in working at a startup than general tech (or, god forbid, completely-general) job boards.
This is true. Also, there's a really neat platform called partunity.com that's strictly geared towards the world of early-stage startups, resources, and job boards. Worth checking out too.