My usecases:

- camera mode. It's not so much about the UI orientation, but it's more about where the shutter button should be located when you pull it out from your pocket to take a quick picture. Preferably the shutter can be easily reached and pressed with the thumb.

- music player. I often listen to music with earphones while the phone stays in the pocket. Whenever I need to manage the music player (skip songs, change playlist, etc), I usually just reach in to my pocket, naturally grab the device in portrait mode and take it out to manipulate the player controls with my thumb quickly and slip it back in the pocket when done.

- email. When I hear the chime of incoming mail, I just want to take the phone out my pocket to quickly scan the mail to weight its importance and whether I should take the time to deal with it now or later. It's quicker and more convenient to do this in portrait mode.

PS: I second all of the items in debernadis's list and bakso's as well.