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.