One of the things people are worried about with that it would allow something close to paid additions/rebuttals to Wikipedia articles, at least if ad buyers were good enough at the AdSense targeting. The goal is to have a neutral article that covers all viewpoints fairly, but then you'd have this little box where whoever pays the most money would get the opportunity to insert a link to their take on the subject.
So all the merchants that can't buy an ad aren't trying to subtly insert ads as content currently? Advertisers only have one choice now--try to get a mention, try to get a link, perhaps as a source--do whatever it takes to get into the content of the article.
If Wikipedia sold ads, they would have the alternative to buy an ad instead; I don't see how the problem of advertisement-as-content would be worsened when given a choice to buy an ad. If anything, I think it's the other way around.
