The PubSub agent syncs the RSS read/unread status of bookmarked RSS feeds between computers using Mac OS X that are syncing bookmarks via MobileMe Sync.
Even though you are not syncing bookmarks between computers, if you have data there, you must be hooked into some RSS, probably from websites, also from Apple Mail you may have an RSS feed. That can be cancelled in Mail Preferences.
Read the client.plist in the PubSub folder to see the sources.
Like a lot of stuff, websites, etc will without asking you, load your computer with cookies and flash objects and RSS feeds. It is electronic junk mail.
I wouldn't get rid of the folder itself but, delete the contents of the Feeds folder, all those numbered files ending in .xml. It will fill up again though, but read the clients.plist to give some clues of where it is coming from.