Indeed and the reasoning was mentioned in another poster's reply to this thread. There are sites which are simply obnoxious in terms of their ad usage and I'd prefer to avoid it initially rather than opt-in to avoiding it on each site. There is, of course, always was the option to disable ad-block on a particular site but the reality is that I frequent lots of web sites, many from when I was younger and didn't care at all about how the site made money thus would never consider allowing the ads. Now I'm trained to not see the ads, not to say I can't change but I think the take away here is that the addition of ads, even if 'non-intrusive' has a significant impact on the UX of a site.
In no way trying to say my view point is 'correct', completely rational or that I'm the best 'web user' for dealing with it the way I have, but rather simply pointing out a contingent impact on the change from my perspective.