They half-ass crack down on them. Not even that, it's dime-assed crackdown at best. You get that little "experiencing issues?" note that the bottom of the screen (which goes away after a few seconds), and an occasional 3-5s delay before video starts.

I explained exactly why I think they are important in that entire paragraph. And the evidence I'm right is the fact that YouTube could trivially prevent ad-blockers tomorrow, but they don't. They've tried it and quickly rolled back the changes. Presumably they lost a lot more audience than expected.