Imagine a wikipedia where each sub-section is paginated to generate more page views like Salon or Newsweek. This is why I think we should support wikipedia directly, so that wikipedia's goal is to make and awesome experience so that we donate again.
Adding ads to Wikipedia would give ad providers the ability to trace your Wikipedia activity. This would be horrible because the average person would not understand that their wikipedia history was being revealed to third parties and this history is often highly personal. For nontechnical users who do not know about this tracking, this would be deceitful. For nontechnical users who do know of the tracking but do not know how to stop this tracking, having ads would severely compromise their ability to use Wikipedia since they must then become cautious of the way they browse.
I know now to open up those links in incognito windows, but every so often I still have to go delete my "viewed items" on the site so the prediction becomes relevant again.
The most common way in which this data is used is targeted advertising, which probably doesn't concern you. Historically this type of data has also been used for price discrimination (which you may care about) and this data has the potential to be used in much more nefarious ways.
Even if we could somehow ensure the data would only be used in acceptable ways, the fact that the tracking goes on without most people's knowledge or approval seems unethical to me, and would be particularly troublesome on an encyclopedia of everything.
This is the same reason why Consumer Reports doesn't take paid advertising in their publications either.
Apologies if you're not a native English speaker or something, but it seems fishy.
For example:
Q: Wouldn't it be better If Wikipedia had ads than planet-wide thermonuclear war?
A: No.
A: Yes.