I really don't get the purpose of the quality score. I think that the marketplace should dertermin the quality. If I'm searching through ebay for a black widget, and I click on a black widget auction, only to find a white widget, then I don't buy the widget.
The same happens on adwords. If an advertiser doesn't have what the searcher is searching for, then the advertiser will eventually force himself out of the market.
I think that's what Google is trying to avoid ... it seems like they want their visitors to get to the best possible destination in 1 click, whether in SERPs or in sponsored ads. That keeps users happy, and keeps them coming back, which protects their market share.