For people with experience and multiple job offers which most likely are the types company's want or at least should be trying to hire, there is no incentive to utilize these services in my mind. While I'm sure there are a few great applicants, I'd guess the quality of the applicant pool is going to be quite low in that the cost benefit analysis for a very qualified person is not so great. If anything, I'd probably see it as a red flag on a company that is hiring heavily from services like these.