First, they're not all bad. I love using planetapache.org as a way to read a wide variety of (growing) blogs on interesting topics. And I get to use it to widen my blog exposure too.
But planet sites are closed. There's no way to join the community or post a single message to the group. The best you get is a contact link to the maintainer, sometimes nothing at all.
For planetapache.org and like-planets, that's probably okay. It's one face of an existing community. After you join the community through published methods, you can communicate on the planet site's forum. However, many planets to a large degree are communities unto themselves, like http://planetrdf.com/. They have their own unique nature and in that blogosphere only a select few may participate.