The ads themselves are not a problem. The problems are

1. It can be security risk. Even Google has ads that redirect to malicious domain
2. Ads that are getting in the way of accessing information. That includes popups, automatically playing videos etc. Hell, one of the major reason why I started using adblock was to block those annoying flash banners.
3. The frequency is just way too much. This leave no money on table mentality makes me not give a shit.