A lone rich donor could cover the budget for an entire year, and many people who could afford that wouldn't even be mentioned in Wikipedia, so there would be little opportunity for any conflict of interest. Even if someone high profile donated a large sum that would only be one profile to monitor for any potential bias.
Here's why I think ads would be a terrible idea:
_________
ADVERTISING
Pros:
More Money
Cons:
Integrity possibly compromised
Contributors might leave
Readers might trust Wikipedia less
-----------------
DONORS
Pros:
Maintains editorial independence
Maintains trust of readers and contributors
Cons:
Harder to raise money
__________
Ads are a lazy solution. I can't think of a single benefit to Wikipedia or its users other than "it would be so easy to meet the budget."