I've noticed, if your a guest, instead of a links unit, you get shown 2 ads, but if your a member you get shown link units? Why? Why not put 2 336x280 side by side, and show it to every one? It's more effective.
I guess thats one of the "benefits" of becoming a member. A Lot of sites, particularly forums, do this.
I think it's rather a good thing. I'm not sure more ads would be an improvement. Anyway, I agree with iceberg "members don't click on ads"
I click on ads all the time. I find that most of the ads displayed on DP are interesting (except those get rich ebooks).