As far as I know, Wikipedia fundraising efforts have been very successful. They got $25 million dollars just last year.
http://wikimediafoundation.org/wiki/Press_releases/Wikimedia...
But yes, I agree, it seems possible for WP to accept advertisement without compromising on its core values. Decide in advance never to give in to such demands, and do not take the funding for granted.
Deciding not to accept advertising might be short-sighted. With enough years of advertising revenue, Wikipedia might be able to work toward financial independence (where returns on investment exceed costs), and build an endowment like the sort that powers top universities.
Also, I don't have to watch any ads! Hurray for Wikipedia!
Even if Wikipedia used the most unobtrusive, low-key advertising system possible, people would still react negatively because Wikipedia is known for having no advertising whatsoever.
maybe? but compare wikipedia's uptime to, say, twitter or reddit. Advertising dollars do not always make for reasonable Engineering decisions. (I'm not saying that advertising causes twitter or reddit to go down often; but they both get dramatically more revenue per user. I mean, dramatically more revenue per user, and they both have terrible uptime vs. wikipedia.)
[1] Jimmy Walles on Quora: http://qr.ae/TL1ku [2] http://meta.wikimedia.org/wiki/Fundraising_2010/Banner_testi...
If you learn nothing else from Wikipedia, learn this: Begging annoys users and isn't particularly successful. He is far more likely to make money selling a premium subscription to corporate users. He can even just say "site is free for personal use, and corporate users must pay $495/year". You would be surprised at how many businesses are paranoid enough about compliance that they will voluntarily pay even if it is unlikely that the site owner would ever find out about their unauthorized corporate use.