One reason is so that the person controlling it doesn't have to be careful to time their hand motions to avoid colliding with an object rotating above the surface.

Obviously. I just think avoiding the arm is easier than trying to precisely move one of those tokens a few degrees around a rotating surface.

I also noticed in one of the demos that it his a wrong note:

https://youtu.be/dYloQf7h0QY?t=60

I suspect if you're not in a well lit room with cool white lighting, there might be issues with reading the correct note.

If you're in a stage setting with flash lights all across the spectrum, it might really struggle to distinguish the notes.

Fun little device though! Could certainly see it in a science museum or hands on art exhibit.

I think the scanner is less precise if it moves instead of reading as a fixed arm

Scanner does not have to move though. Just taking an image and analysing that could work. Maybe from underneath. It would be nice to have large surface without strict grid to play around placing the objects.

I get the turntable reference, but if you can’t scratch it’s sort of pointless form.

You're right. The turntable would have a lot more angular momentum. It has to be easier to maintain precise RPMs.

You need to have two, a soundmixer and a DMC turntabelist.