Also, to clarify something else... ad CTR is usually based more on the advertiser themselves than anyone else. If two advertisers create their ads, they will have different CTR's, even when on the same content. There have been ads that run a 0.00% CTR because they flat out suck, and others than convert 10%+. When you bring revenue into this as part of determining how much each writer's "value" is, you're not only basing it on THEIR work, but also the advertisers', something the writer has absolutely no control over.
The author does have some control over the advertisers. For example the write can write about somethng advertisers pay $100+ per clickthrough on, or something advertisers pay $0.05 or less per clickthrough on.
Often the high paying ads will try to word themselves in a way that will discourage anyone but truly interested parties from clicking through, such as making it clear their product costs money, that they are not offering any freebies etc. Whereas topic paying low rates might well want every click they can get, so will say stuff intended to get anyone to click, since they might well be sending the traffic to a page that will from there sort out the traffic and push it into higher-paying variants of the topic or higher paying related topics.
Also once an author sees the kinds of ads their page gets assigned, they could adapt their text in ways intended to create more interest in some of the things they have seen advertised on their page.
-MarkM-