I don't think it is that clear, actually. If I sell a car that is as good as the Ford one you searched for, but 80% of the cost, wouldn't you want that ad to appear when you searched for ford?
If the discount car is relevant, then I want the search engine to show it to me whether or not you happened to pay them for an ad. And vice versa if it's not relevant. As a user, I don't care who is getting paid by whom; I care about seeing relevant content.
Respectfully, I’d want the car you mention included if I search for “full size sedan” or “alternatives to ford”, and not to appear if I search for “ford”. This is a pipe dream of course with the way AdWords works, but that is what I prefer :)
