In essence, my argument is this: because at their core, they are an unbiased source of information, the less controlling interests involved, the better.
To generate revenue from ads is to "sell" a piece of themselves. Equate it somewhat to raising money from investors. They now have much more revenue, but at the cost of some of the control of their company. Ads certainly don't take away control in the sense investors do, but depending on the revenue from ads in any way is an extremely slippery slope.
How about only showing the ads until each month's (or the year's) costs are met?
That way there's no temptation to try to derive a profit from it, and at worst you end up with ads on Wikipedia for a couple of months out of the year (kind of like you get now).
To generate revenue from ads is to "sell" a piece of themselves. Equate it somewhat to raising money from investors. They now have much more revenue, but at the cost of some of the control of their company. Ads certainly don't take away control in the sense investors do, but depending on the revenue from ads in any way is an extremely slippery slope.