Banner ads are an almost invisible inconvenience, and a small price to pay for an othewise free site. Besides, they are pretty much ubiquitous on web pages now, so seeing them on UU isn't anything special. In fact, banner ads are so common, I honestly don't even notice them when here.
The bigger concern was a couple of weeks ago when there were pop-up and hijack ads that actually did interfere with the site. Those ads are not only hideously annoying, but some of them were also malicious, and more than one UUer (myself included) noticed their security software starting to respond to them. If those ads remained, I would have simply not returned here. Personally, I don't think the UU guys were responsible for the malicious ads, and credit to them for getting rid of those pests.
As far as the banner ads, it's a way for UU to make a few pennies here and there without directly charging the members. And that is totally cool.