I think back when the poker boom was really kicking in, the $10 free deal worked out a lot better because it got a lot of new players in who were apprehensive about credit cards. Hell, it got me playing originally at Royal Vegas when I'd never played poker before, and I ended up going professional for awhile. From the free $10 I got from Royal Vegas, I probably ended up generating in excess of $5k rake for them, quite the investment
These days though, I think you're more than likely just going to get bonus whores etc who take it. Of course all they have to do is generate $10 in rake for it to be break even for a room, so in theory at least it's not a bad idea.