One is where you are searching something and they show you relevant offerings. Here, people are often happy to see the results, as they can move them toward their goal.
The other is where you are trying to do something (read a blog post, watch a video) and they want you to do something else (read about somebody's product, watch a video advertisement). Here many people are resistant, as it takes them away from their chosen activity.
My understanding is that Google makes most of their money from the first kind of ad, and that ad blockers mainly block the second. So I don't think this is a very big deal for them. In Google's shoes, I'd happily let people run ad blockers if they wanted. One, those people were unlikely to click on ads anyhow. And two, it's dangerous money. For their long-term success, they're much better off if they're doing things that customers actually like.
It's dangerous when companies get hooked on money that requires them to work against their users. Look at how much trouble TV networks have had adapting, for example; they just aren't clear on how to make great TV in the same way that places like HBO are. Or look at the sad, slow fall of Yahoo, which has always thought of itself as a media company. They were never able to do much with customer-focused acquisitions like Flickr because they never could quite understand them.