Responding to the topic title,
I found this forum from a website which linked to the portrait mode thread and asked people to give their reasons for portrait mode.
My main reason at the moment is one handed use.
I was injured playing sport earlier this year and walk around with a crutch now, so stopping in the middle of the street to open a keyboard in landscape just to type ok in response to an email, SMS or IM is not really appealing. This is analogous to holding a shopping bag or basket, or carrying your suitcase in an airport etc