Why is financial gain given such a bad rap compared to just about any other type of gain? What about those who exploit Wikipedia for their own political gain, or heavenly gain, or for some misguided utilitarian gain? I'd say the bigger threat to Wikipedia is from those who have a goal other than personal financial gain. Time and time again I see articles ruined not by corporations trying to sell their goods but by idealogues trying to push their utilitarian (or religious) theories.
We're fighting those threats on a daily basis, and generally winning (we win battles, we'll never win the war, it's the price we pay for allowing anyone to edit). Adverts are a different kind of threat - they're a threat from the inside. It's much harder to fight internal battles than external ones. Of course, the people that are actually in favour of adverts on Wikipedia are in an extremely small minority, so there isn't really a threat at all, it's all just sensationalism.
I've never understood the strong aversion to money seemingly held by some Wikipedians.
Few people are averse to money, it's money at the expense of the values Wikipedia is built around (freedom of knowledge, for the most part) that people don't like.