This seems like a bug to me. I have imported a couple of RSS Feeds from Safari. At first everythig seems to work ok. The feeds get downloaded and the user has the option to make mail.app the default RSS reader. After a couple of minutes the RSS option on the left side of the screen disappears! I have to quit mail.app and start it again. After the while the same happens again and again. I have confirmed this on both my computers!