The ads themselves are not a problem. The problems are
- It can be security risk. Even Google has ads that redirect to malicious domain
- 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.
- The frequency is just way too much. This leave no money on table mentality makes me not give a shit.