In an attempt to maintain (appearances of) neutrality, we'd have to spend quite a lot of effort controlling who gets these advertising slots on contested topics; that's going to eat into the profits quite quickly. It's not a simple matter of "and now, open the money pipe"
Actually, it just might be that easy. What if we opened it up in stages and get feedback along the way:
Step one: 100% optin advertising where the user has to turn it on. Step two: put ads on search results, or give folks the option to "Use Google to search wikipedia" with the ads going to Wikipedia--so again, opt in. Step three: if steps one and two do well, PERHAPS put ads on pages by default. In fact, Wikipedia could run ads up until the point it raised $50M and then turn them off and live off the 3-5M that would turn over in interest!
Step one is a total no brainer... i can't think of one reason to be against OPTin advertising.... can anyone?
You haven't actually addressed the issue to email you replied to was talking about. We're not saying users won't like ads (although that's probably true as well), we're saying that inappropriate ads could cause serious problems, so we would have to control what ads we show, which causes even more problems.