I think that the term is called in the car world 'Brand Engineering'.
Most paysites thrive on this, they have 1 big members area with sections and they just attach frontends to them, actually, there are certain programs around that let you profit from this concept in various ways.
I can't see anything wrong with those sites having the same galleries, and you don't get affected since, as Steve said, it's a pay per sign up proggie.
Peace
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