I feel that one of the reasons that RSS became so popular was that it allowed readers to avoid all the extra fluff on a website and get right to the content, thereby increasing the amount of content you can read in a day. Inserting advertisements into those feeds dilutes the value of the content. If, like in some low traffic feeds, the advertisements out-number the actual posts, it can become a justifiable reason to unsubscribe from the feed. I think that many content providers are going to have to learn the hard way that social media (as bloggers are sometimes called)(as opposed to mainstream media) allows for very fickle readers. Contrary to what most content providers think about themselves, very few feed sources are "valuable" enough to be able to keep their subscription levels while annoying their readers at the same time.
In any case, how long before someone writes an aggregator that filters advertisements? Do we really have to join that arms race?