"One-handed" actually means "one-thumbed" and my lone thumb can't provide decent control. If at all possible... I use two hands like you described: one hand holding the phone and the other hand's index finger providing the input.
I can send a text message with one hand (one thumb) thanks to the keyboard being at the bottom of the screen.
But there is soooo much more I do with my phone. Pinching and zooming? Needs two hands. Multi-finger gestures? Needs two hands.
And if I'm banging out a lot of text... I flip over to the landscape keyboard. Two hands.
I'm ready for a larger screen... my eyes are ready for a larger screen. My pockets can handle it.
My lone thumb? Not worried... he didn't get a lot of solo action anyway!
Totally with you. The phone already has too many functions better served by the index finger or two thumbs, than with one thumb.
Notice Jobs showed the first iPhone with the INDEX finger graphic...not a thumb. ;)