If ads were just gifs or pngs that loaded from the same site, did not noticably impact page load times, and were displayed alongside the main content (analagous to ads in a magazine) I would likely not bother blocking them (and blocking would be more difficult: how would the blocking software know that they were ads and not part of the main content of the page?)
I block ads when they do things like take over focus, are distractingly animated, slow down the page, overload my CPU, etc.