Earlier quoted context omitted.
You may substitute "commercial ad" every time you see the word "ad" in discussion about this, if you'd like to be picky.
I HATE non-commercialism. It's like saying that a charitable hospital can ask for money, but can't run a cake-stall to raise it. Charities end up spending a huge amount of money soliciting donations. Wikipedia's lucky, because they can nag everyone on the internet for essentially no fee, and even partly deny service (by offering a gimped service). The Red Cross can't give disaster victims a nag message before helping…
You can argue that users will be able to distinguish between the ads and the content, but Wikipedia serves a very diverse community -- children, the elderly, people who can't read very well, people who are more-or-less computer illiterate. If even a small percentage of them are confused by the ads, Wikipedia will have failed in a small way.
(FWIW, I've seen technically-literate peers get confused by the current banner too, thinking the person pictured was the person the article was about.)