> But I don't get any $ from it unless they sign up on MY site, right? Since there's no sharing mechanism.

Exactly.

> So I don't see how joining in would benefit me - if anything I'd lose a bit of revenue from people who would have paid and now find they don't need to because they're signed up for some other product which I have no hand in and no revenue from?

It would not benefit you if the average person paid for multiple free software projects. In that case, they would only have to pay for one instead of multiple.

I don't think that's the case though, so this solution should make more people pay for free software and that should benefit the developers on average.