You'll have seen sites that completely refuse to allow you access when they detect an ad blocker, and frankly these sites I'm quite content not to use, there are simply so many sites out there that well.... next.....
Often I'm sure sites like these or the slightly more sensible ones that request you don't use an ad blocker, are very probably motivated to protect their ad revenue and probably see people who use ad blockers as people who are stealing their content.
Alas there is a far more important reason to use an ad blocker, aside from the privacy sapping, the flashing attention grabbing and annoyance, its simply one of security. In an ideal world an ad agency would carefully vet the code in the adverts before hosting it and its artwork on their own servers, unfortunately it would seem that many ad agencies don't seem to do much in the way of due diligence and sadly have become one of the main vectors for malware.
Its all very well and good website operators moaning about people stealing their content, and complaining they can't get by on their dwindling ad revenues when they don't ensure that what is served on their site had been properly audited. There are many other ways to revenue from a website be it "premium" content by subscription for the truly fanatical, competitions sponsored by the prize donator, the list goes on. Website owners looking to earn a living from their website really do need to diversify their income streams, then maybe we won't have to suffer adverts creepily chasing us from website to website and our viewing habits might just become a little more private....