This makes the "Twitter-like" system less "Twitter-like". Don't get me wrong, I see your point, and I can think of a technical solution (point to OPML feed collections instead of feeds), but the further this gets from the original popular thing (Twitter) the less support and momentum it's going to have, imho. On Twitter it's one stream per user, and that keeps things simple. And simplicity is a huge part of the value add of twitter.