To be honest, I normally pay them no mind in the first place, which might be overall more harmful.
But an ISP blocking ads is very dodgy... An ISP tampering with web pages at all is wrong - I think an their job is to deliver only, and exactly what the web server gives them to the person who requested it, who is then free to choose what part of the page they see. All the more reason for more web sites to use SSL/TLS.
When you block an ad on the web it's not being counted as an impression and the content provider is not being paid (if CPM).
They're very different in terms of money-flow. It doesn't mean devaluing an advertiser is better, but they're two different outcomes, even if it just means "no ads" to user/viewer.
it depends on how the ad is actually served, but i would assume that a smarter adblocking tool would "fake" the ad request, so that an ad provider would not be able to tell the difference between a user seeing the impression or not.