While you have to take some of what was said in this article with a grain of salt (considering the source is an anti-spyware company executive), it does bring up an interesting point.
Adware would not be floating around the net, and penetrating unsuspecting users’ machines (Windows, that is), unless there was money to be made.
How much money is actually been made could be subject to enormous debate, but I am not going to get into it with Richard Stiennon of Webroot. But like spam next to it, there is no argument here – there is money someplace. The article above notes that the figures could be akin to estimating porn revenue – you know it is big, but who is actually getting what, and how much, is nearly impossible to estimate. And lets not forget, hackers have made the transition from working for pride to working for dough, and many are playing the spy/ad/malware game.
With the problem growing (some consider it still nascent), the economics of spyware, meaning both sides of the coin (the implementation of bugs, as well as their removal), is going to keep players in the space for some time to come.