Hey Gene,
As there are good marketers, there are bad marketers too. Many people ask for a review copy, and never review. Thats not the bad part(atleast). The bad part what I have seen is, they SHARE the product on blackhat forums. This is bad.
So before you give away any review copy, be sure you ask the guy these questions:-
1. How is he promoting your product? + Proof(list,review website,ads or what)
2. Does he have a list? How big and Proof (usually a snapshot of his email account)
Also, before you give out a review copy, it is correct to search for the name on Google. Most of the time you may not find it, but sometimes you do.
And, in a launch that I had observed(name not to be mentioned), the author was not giving a FREE review copy, but for the actual product of $39, he was giving away review copies for $19. Another safe way.
Hope it helps