My thinking is that when a surfer joins an AVS site one of their main reasons is that they get access to thousands of other sites. So they figure they'll never have to spend another dime on porn again
They are wrong of course, lustful human nature always demands more no matter how much you get, but they've made a bit of a commitment and they do stick around a very long time, and mostly surf within the network. So having the right ads inside your sites for camgirls, dating services, and videos on demand gives you a marketing edge for when they do get tired of what the AVS offers.