There are definite exceptions. But do you really want bookmarkers from review sites joining your sites? The only kind of surfer I can imagine bookmarking a review site would be the ones constantly joining sites, quitting sites, joining sites, quitting sites. Good for the review site operators, not so good for the affiliate programs paying $30 per join on a surfer that is most likely isn't going to stick around long. They're not bookmarking review sites to come back for free porn like they would with a TGP.
A surfer that frequents TGPs probably doesn't join paysites very often. That's why they're surfing the TGPs enjoying the 15 second clips. One day they see a girl that really sparks their interest. They HAVE to see more of her or want to see more of the scene they just saw. So cock in hand they join your paysite. And now they're introduced to something they're not used to seeing. Full videos. High resolution. Huge selections. Feeds. Pictures. Cams. everything. Because this is new to them, they're more likely to convert to full and stick around.
A surfer that frequents review sites on the other hand... They've seen what paysites have to offer. They've got a harddrive full of videos they've ripped from other sites. The bells and whistles of a paysite aren't new to them. They know the tricks. They'll cancel their trial before they even login to the site.
Again, there are exceptions. You really have to handle review sites on an individual basis to see how they perform. If the surfer comes to the review site from a search engine and joins, that's good. If they're a bookmarker that tries a new site every week, that's bad.
That's my take on review sites vs tgp sites.