Don't underestimate the psychological value of reminding people that they've done something they want to feel good about. As long as it doesn't add to page load time, a reminder saying "thanks for donating to support this page via Google Contributor" in place of an ad helps keep this program in people's minds, keep them willing to pay, and help them think about it at some time other than when they see the cost charged.

Indeed. It's a little thing, but even a small icon at the top of the page that tells me "I'm supporting this site with cash, I'm awesome", would be a small self-esteem boost; a reminder that I do reward the author whose content I benefit from, and a nice feeling that he's getting my money directly. Just keep it unobtrusive.

Come to think of it, where I'd really like to have it is the browser - say right there in the address bar, next to the RSS button (please, please bring it back!). This way it will be unobtrusive, in a consistent place, and won't interfere with the page layout. Win-win for everyone.

Presumably because the specifically-sized iframe might be part of the site's design, and Google doesn't want to muck with it if the site designer might have been too lazy to foresee the problem (because Google would be the one getting blamed.)

They could obviously tell the site the ad should disappear with a JS event or somesuch, though, at which point the DOM can rewire itself.

It might be for technical simplicity: this method requires no more work from the site designers, who are often contractors or otherwise expensive, and off the top of my head I don't know how they could do this kind of disabling on the client side without allowing someone to make a plugin that tricks the site into thinking it is in addless mode.

The pattern is what worries me most. It should be understated and transparent. More visual noise seems like a distraction.