Could you imagine using your PC or phone using just six buttons and a rotary encoder?
I do remember using my phone via just a rotary encoder.
No doubt having the touch screen is a nice extra; and if I should find it too fiddly, I can always use a stylus, a mouse, or the knobs and buttons for all routine settings.
But the touch controls take room on the screen, and on the DHO 800/900 with its already small and 16:9 screen, they leave a relatively narrow "letterbox" area for the trace display. So one does pay a price for the touch funtionality in terms of screen real estate. Hence my questions around usability.