The problem with running adverts on any basis is that the people who were inclined to donate money to keep the site running, the people who think about how much it must cost and what must go into it and go "Hmm, I like this place, wonder how I can help," will just disable their adblocker and earn you a couple pennies, when they'd happily give you a fiver.
When we cut off all ads, even the opt-in ones, donations tripled. It was like people suddenly remembered that clicking doesn't do anything.
