Google's pay-for-results ad model

Google is playing with an interesting new ad model, in which advertisers pay only when someone actually buys a product from them.

I've always wondered why sites don't try a 3-tiered system:

Put up a text ad that says you can get more information, including an offer of a free or discounted product, without leaving the site you're on. People click on it and a small window pop-up plays a video ad for your product. At the end of the ad, you print out a bar-code coupon for your freebie. The advertiser pays one price for this click. This generates brand awareness.

During the ad play, there's a link to the sponsor's site. If you click on it and go to the site, the advertiser pays another fee. Deeper brand awareness.

Then if the person buys something from the site, the advertiser pays a third fee for the referral.