I think the confusion may be that with BlackBerryID, your BBM contacts are automatically updated with your new pin when you change devices and then log into the new device using your BlackBerryID.
Since the BlackBerryID is an email, users can also use that to invite somebody to BBM rather than having to know the specific PIN of the device they are on. I am not sure if you invite them via the BlackBerryID if it just sends them an email invite or actually looks up the PIN of their device using the BlackBerryID and sends an invite via PIN.