Let's say you have 500,000 users, and each gets 10 ad impressions a month. Let's be honest, 500,000 users is a LOT, that's more people coming to see your site, than most local broadcast stations, yet they really don't make you that much money.
That's 5,000,000 views, at CPM of $2, that's $10,000 a month.
If we used that same 5,000,000 and switched to CPC, we'd get a CTR of about .1%. So that 5,000,000 views would net us 5,000 clicks. Let's be generous, and say each click is worth 50 cents. Bam, you only made $2,500. So your income has just been cut in 4. And that $2,500 will twindle since ads don't really change, so a user will only click them once.
If we used the same 5,000,000 views as CPA, the result will be even worse. People just don't like pulling out a credit card(at least not without checking other sites for a better deal). If your CPA is 1 action per 25,000 views, and you get $10 for each transaction, that's $2,000 worth of revenue. Bam your original $10K has been cut in 5. And remember, you'll need to find brand new offers every month, since the ads will get stale.
With an income of $10K/mo, you can focus entirely on your content, but with only $2,000-$2,500 you have to go pick up a 2nd job.
This proposal is probably a good idea for big sites like CNET, since that would kill all the smaller content providers and drive them out of business.