I don't think your comparison is completely accurate. I get a lot of great submissions at AdultCore, but I'm sure the amount and quality of porn I provide doesn't compare with a good paysite. It's more like McDonalds is selling burgers on one side of the street and I'm giving fries away for free. Sure there are some guys that will just eat 2 orders of fries and go home happy, but alot of people will grab my free fries and head over to get a burger and Coke to go with it.
And the important thing that TGP's do is allow the little guys putting up a paysite to get their piece of the pie. I mean anyone can promote their sites for free on hundreds of high-traffic sites and all they have to do is submit.