How much of your monthly bandwidth allowance are those ad-laden sites consuming?
And then there’s the matter of one’s browser slowing to a crawl…
http://adblockplus.org/blog/anatomy-of-ads
For the record, I’m certainly not opposed to moderate amounts of advertising on websites and I tend to only activate adblock for those sites whose design is a fit-inducing visual nightmare, whose adverts interfere with my ability to understand the content, or else for those that have poor performance due to the number of connections to advertisers (described in the adblock post linked to above). But if you play nicely, I’ll leave your ads in place. Quid pro quo.