But why does that destroy the purpose of a touchscreen again ?

I mean its never been mentioned that a touchscreen has to be always facing the user at all times. You could very well close it down (added protection for the screen) and pop it up when you want to use it.

I have used a Clamshell Zaurus extensively and not once have I felt the clamshell defeating the purpose of the touchscreen on the Zaurus.
Of course the added bonus with the Zaurus was that the clamshell factor could rotate and lie down with screen facing up in portrait mode too as well as sit down laptop like in landscape mode.