My thoughts (biased as they may be
) is that ad networks do provide a valuable service to the adult industry.
You list a lot of reasons affiliates prefer to do business with ad networks rather than directly with programs. I don't know about those, but I do now that filtering traffic through a proper ad network can be beneficial for both sides.
For one, advertisers get quality hits from countries that are allowed to join their program. So they zone in on their target market
Second, affiliates get to monetize the remaining traffic that would normally just be wasted.
As nick says, if traffic is bought by affiliates it is to get a higher number of people viewing content - promo content from programs so in this case, the affiliate is sort of acting as an intermediary between traffic sources and the sponsor