I don't think a lot of people realize the feature is there. For clients its a referal that is available on demand w/o having to wait for the provider they are looking to get together with to contact previous provider (hoping to remember something unique about the client to give him the green light). But then again some providers may be reaaly strict about screening or TER averse and not want to use it at all. I like it. For a guy that doesn't hobby often it helps out.
If you whitelist them it may be a few less calls to you from some strange girl asking about some strange guy that you might remember.

Try asking your clients to see who would want it.