For the most part i don't mind ads, and i can totally understand why sites would host them, and i often do listen to their appeals and turn off my adblocker for that site. I don't even care much about what they're showing as long as it's not explicit (it's not like i ever click them anyways), so why do i have uBlock installed? Simple: Noisy ads. The ones that make sound, especially automatic ones. At some point i'll have to try and see if they're gone for good, and if they are, great, i'll let the sites have their ad revenue because it's not like it hurts my experience. However, it bothers me intensely to suddenly have sounds playing in my headphones that i didn't approve of, especially when i'm listening to other things, be it music, games, videos or whatever. I suppose i could be doing a blacklist approach instead of a whitelist one, of course, since that wouldn't hurt sites that don't host noisy ads. I don't think that's too unfair, unless sites have absolutely no control over which ads get hosted, in which case it's unfair that they don't.
Also, as Cript says, viruses are a concern that needs addressing. Bandwidth isn't a problem for me, but i can see it being a problem for those who don't have the luxury of unlimited bandwidth, especially if the ad designers are being slobs with their bandwidth usage. If the advertisers want more views, they need to stop letting those kinds of ads through, because that's what makes people install adblockers.