First things first — whether you create an account or not, the functionality is exactly the same
However, if you log out of a guest account (e.g. if you get a new phone and have to reinstall the app), you'll not be able to log back in with the same account. You'll have to create a new Metacast account, which means that you'll lose all your playlists, podcasts you follow, listening history, and other data associated with your account.