It's simple (Score 0)

Ads are a turnoff for contributors because they don't want to feel that the content they create is enriching advertisers. Of course, the byproduct of enriching these advertisers is that it helps Wikipedia. But the best thing about Wikipedia, as opposed to 99% of all other websites, is that it DOES NOT CONTAIN ADS. Advertising is the slipperiest, most insideous force out there - once you let it in it's over. Trust me, I work in advertising. I posted a comment on Digg originally about the fact that allowing advertising for Virgin Unite is simply brand advertising for Virgin, a company whose entire business model is based around a brand identity and very little else. It's a shame that they've managed to play Wikipedia and get 1000x more buzz out of it than they paid for. Good business move for them, bad for all of us. Wikipedia needs to be dogmatic about advertising I'm afraid.