To answer the original question, some of these M21 could be floor models/demo units. And businesses should demo the newest model, with the new screen.
Anyone that still has the pre-MDx upgrade, it is well worth the money to upgrade it. Based on my experience of pre-MDx and with the MDx in an M3.
I currently have an M21 and love it, part of me wants the new display, but without any sonic improvement, i'll hold off.