I deal with this all day every day working with advertisements. A lot of money is spent trying to detect "bad users" and/or "bots" (usually the same thing). I'm talking hundreds of thousands of dollars, if not millions a year in some cases.
I'm actually working on developing a system to track browser analytics and usage to detect if it's a person on the other end or a bot.
The quick solution of course would be to have a captcha when viewing ads on sites so the advertiser could confirm it's actually a legitimate user, but there are users that are doing everything they can to not be tracked/or view ads, so what incentive do they have to confirm they are a human just so they can be targeted for advertisements? That's why there are companies trying to work behind the scenes to see if the browser is a legitimate session, or a bot session.
Companies looking to buy advertisement space are really honing in now on bots, because it's become such an issue where server farms are set up that will automate views on pages to inflate profits, or like in the case of the company that runs this script on NYtimes, to see if the user is viewing the page through a legitimate viewing session, or if the user is running software in the background of their computer pushing page views automatically.
I could probably talk all day long with this, but advertising is a huge HUGE market. There is little to no day-to-day talk of the users that are running ad block on their computer, it's a low percentage of the actual users we are running into. The large talk is the people that have created botnets of hundreds of computers to push thousands of fake impressions and how to handle that.