BoingBoing is ably demonstrating a looming difficulty with webfeeds and advertising. BoingBoing is in the Creative Commons, using the Attribution-Commercial license. If I "remix" their feed, for free, and remove the ads, I'm guessing they wouldn't be happy.
The bigger tarpit is a potential clash of commercial interests. What if some other commercial entity wraps aggregated webfeed content with contextual ads that are in direct competition with the ads in a feed. Think trying to make a buck off of MyYahoo! The content providers probably have the legal high ground and are fair in demanding some kind of payback. But then middleman or client side wrappers and remixers have to negotiate hoards of licenses. Or bend over for centralized providers like Moreover. Strikes me as a mean disincentive to provide high quality aggregation services, which cost money to develop, deploy and support.
Maybe there'll be a middle tier of "prosumer" aggregators, desktop one time fee, or web based subscription supported. These guys will innovate on useful but not wildly popular features where you can get paid by not conflicting with the ads.