But they've now rolled out Adsense for Domains anyway..
Besides, surely it's the context that matters more than the content of the page? If you sell diapers for monkeys (let's say!) and your ad appears on a parked domain called monkeydiapers.com and someone clicks on it, that visitor is probably more likely to be a buyer than someone who clicks on the same ad on a comedy Web site, right?
I agree with your example regarding propensity of the person being a buyer but the advertiser may not want to show his site on a parked domain for, say, branding purposes.
When I first noticed that I was getting a ton of worthless traffic, and paying several hundred bucks a week for it, I dug into the AdWords UI, and found only one option to disable, "Content Network". Soon after that, they added the ability to place ads on specific sites...but not the ability to say, "anything except spam sites, including Google's own spam domain hosting service". Of course, Google doesn't talk about their own spam domain hosting service much. But, it's clearly Google being evil. (Apparently most Googlers aren't even aware of this "service" from Google, so I guess it's kept quiet internally, as well.)
The Content Network
and then by "platform" .. Desktop vs iPhone + smart devices.
You can set content network to targeted though, but I don't see any way of totally opting out of parked pages. I agree with you, however, that this is not necessarily what advertisers want, although ultimately I think it could work out if Google maintains quality.