[callmeal]

>My guess is to prevent ad-fraud by website owners. It's a lot harder to detect fraudulent clicks/impressions if all data are routed through website.

Isn't the solution to that problem a flat rate fee (similar to how advertisements on tv, newspapers and magazines work)?

Instead of a pay-per-click it could be a simple $X dollars and your ad will be visible for Y days/weeks.

[uponcoffee]

I don't see how that would work. If my site gets zero traffic, would I still get paid a flat rate to 'serve' ads? Pay per impression/click works to pay proportionally to individual site traffic and the extent of a campaign.

The current solution is effectively a flat rate as far as an ad campaign is concerned: impressions/$

[michaelt]

People would either (a) pay to place ads on sites they knew had a decent amount of traffic just from reputation, or (b) would hire ad-buying companies which made it their business to know what different sites' ad space is worth.

Needless to say, this could be inconvenient for the adwords-make-me-five-bucks-a-month scale sites. It'd work out OK for the New York Times-es of the world though.