I use a Passport external with my MBP. When I plug it in, Time Machine starts and does a backup. When done, usually a few minutes, I just eject and unplug. When I open the Time Machine preferences, the Next Backup line says: When Disk is Connected.
I have not had any problems using it this way. In fact when I converted all of my stuff from my Macbook to the MBP I just used Time Machine and it worked perfectly. I just make sure I just completed a back-up so it was up to date.