My issue is that it's an inefficient use of human resources because it clutters the interface. If you're looking for the answer to a question, you have to post in multiple places and/or search/review multiple communities to see if the question has already been answered. For low-traffic communities the replies get split, suppressing topic participation. For high traffic communities, stories/links that get posted to the "same" community on multiple instances clog up personal home pages and - in the case of large participation - clog up the top feed.

Again, imho, there should be a way for communities to aggregate or sync across instances and be shown as a single feed, like a symlink to multiple folders that is treated as a single location for end users. I realize this causes moderation concerns. I still think its better for the participants.