I think there's a reason that most handheld devices with similar form factor (elongated rectangular shape + slide out landscape keyboard) defaults to portrait orientation with the 'optional' or secondary landscape mode that gets triggered when the keyboard is slid out. I'm no ergonomist specialist, but there's something about the default orientation that you'd get when you put that form factor in your hand, and how that is just one step away (one click, one swipe, one whatever) to activating the device to interact with it without having to involve your other hand.