Phil Ringnalda wrote recently about the potential damage to the blogosphere of planet-style aggregators. I've thought of another problem. If you're being aggregated onto a planet site then none of the other people on that planet will link to your posts. Why should they? After all, it'll just look like noise when it hits the aggregator. This could have serious repercussions for your GoogleRank.
This issue doesn't affect me too much since, as far as I know, there are no planets aggregating the whole of my blog. Planet RDF just takes my RDF category feed and I don't aggregate myself on the two planets I run (Agile Planet and Planet Web 2.0) because they're designed to be my reading lists not a publishing mechanism. (Actually I do partly aggregate myself on both - on Agile Planet I aggregate a category of posts from this blog but they're just for status messages and I write on the Silkworm group blog which is aggregated by Planet Web 2.0)