I did notice this recently and have been having to uncheck the box on all the new PCs I install it on. While the ads are "unintrusive" I have a media PC where I've turned the font up to huge to be able to read from the couch. Some search terms will cause maybe up to 4 different text ads to show up and that will effectively block out the actual search results when your font is large.

I'm glad I found out about this though. I always felt vaguely guilty unchecking it, because I thought it was maybe AdBlock trying to support the "Good Guys" of online advertising, but if Google themselves are paying for it, I no longer care.