except they are running ads right now.. of an ugly guy's face
It can go too far the other way, though. I heard that some of the "Pink" products gave a fixed fee to breast cancer, so buying their stuff wouldn't give any money. Others gave a tiny portion.
I'll concede that it's a good model for wikipedia - spend nothing, and nag users for donations. But it's not a great model in general. But it only works as a replacement for micropayments, and that's where unobtrusive advertising works better.
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.)
It's much more difficult to detect and evaluate bias within the text of an article. There is plenty of this on Wikipedia now, usually because of partisan or ideological edit wars on lightly read articles. And it's getting worse as more and more barriers are erected to casual or first-time editors.
It is almost impossible right now for someone viewing an article for the first time to improve it, even with cited sources. Regardless of quality, the changes are frequently reverted quickly by a bot or someone squatting the article. To get a significant addition or change to "stick" requires a large time investment and knowledge of the arcane rules of appeal within the growing Wikipedia bureaucracy. Result: most people don't bother.
This, to me, is a much larger problem of bias than what you describe. I actually think ads could help this problem by decoupling the content from the money. Right now a relatively small group of people are the most powerful editors AND the largest donors. Wikipedia is at risk of becoming captive to a set of people who think it is "theirs."
Business is hard, and it's a different kind of hard than charities normally deal with. Being good at 2 things is more than twice as hard at being good at one thing, so I think charities are wise to generally stick with being non-commercial.
Other objections are that the advertisers would be the ones paying Wikipedia, and thus have the ultimate control; they can threaten to reduce advertising revenue in order to get objectionable material removed, they would incentivize people to put more advertising-friendly content that would draw people to lucrative pages and entice them to click on ads.
Ads are also a security and privacy risk. Advertisers who can inject content into pages can do varios nasty things. And advertisers tend to track what you do; tracking your Wikipedia browsing habits would likely be quite lucrative.
One of the worst things about the internet today is that so much of it is funded by advertising. This gives companies no incentive to act in my best interest; their only incentive as far as I am concerned is to get me to look at their page, so I can look at and click on ads. They are beholden to their advertisers, not their users.
The same is true of contributors as is the response - get different contributors/advertisers.
> They are beholden to their advertisers, not their users.
They lose advertisers as soon as they lose users, so users control their advertising revenue.