There isn't really an incentive to opt out. The constant referencing of ads is a bit misleading. It's just that ads happen to very often be the slowest loading content on the page, which causes lots of annoying reflows. Really it's any image, video, or any kind of embedded element whose size is not known during the initial page render. Ads just happen to be a serial offender for this, because they are typically served by 3rd party networks, so the page cannot reserve a particular bounding box as is usually done with images.

It's not that sites are intentionally scrolling ads into view. In the rare cases where they do that, scroll anchoring won't make any difference anyway. Usually what happens is that the content just changes position randomly, whether or not the ad is even within the viewport.