Could someone please explain why advertisers do not re-use functional domain names to defeat domain-based filtering? I always find it fascinating that they still use such obvious ad-only (sub)domains to host assets.
Anyways, yeah, some ad companies are starting to do exactly this. They're serving ads up from the domain/website the ads are displayed on. Host files are completely ineffective here unless you've already previously spotted the ads and manually blocked them (and provided they haven't changed the file name of the ad/picture since your last visit).
But the ad companies doing this are tiny. It's quite likely you've never been on a website that has enacted this method. Most ads on the 'net are served up from google's adchoice/adsense program. Once Google themselves start doing it, it's game over for adblockers and for host blocking. I heard through the grape vine that they're actually working on this very issue (the issue of ad blockers).