[wpietri]

It is a very common problem in ad-supported content businesses. Reputable newspapers and magazines work hard to make sure advertising doesn't influence content but it's always a struggle.

For example, suppose that Coke buys $10m of ads a year. It's small money to Coke (with a $1.6bn ad budget) but that's 60% of Wikipedia's annual revenue. Then suppose that there's a press furor over the "Criticisms of Coca Cola article" and Coke threatens to withdraw their ads. It would be impossible for Wikipedia to be perfectly chill about maybe having to fire half their staff.

It might even be less direct than that. Might people make more content that garners high-paying ads, even if the quality is low? Could editors be tempted to make the content less accessible (or harder to tell apart from the ads) if that gets more click-throughs?

Accepting ads mean there is always a conflict of interest between getting paid and serving readers. Even if Wikipedians make the right decision 100% of the time, readers will still be more suspicious, and that's the last thing Wikipedia needs.