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.)
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!
[1] Jimmy Walles on Quora: http://qr.ae/TL1ku [2] http://meta.wikimedia.org/wiki/Fundraising_2010/Banner_testi...