This is really unfair statment.
the first mia was introduced in Maya didn’t have a frame buffer capability. Thus the introduction of mia_X at the time users had to write their own custom shaders to output these passes
Moving to 2009, Autodesk introduced their own passes system that doesn’t support all the passes from the mia (it was geared towards the regular maya shaders) thus the reason for the mia_x_passes
Now you will be surprised how many of the big studios still uses old vrs of any application, weather its Maya , mental ray or whatever
The entire pipe line was build on a specific Vr, upgrading sometimes can take years.
I would really recommend reading the documentation of every release > under the “what’s new section”
Hope this helps
All the 3 shaders do the same thing.
Theres no need to have 3 versions of the same shader. Could be only one, with passes capability.IMO.
But to avoid confusion, and to keep compatibility, mia material, and mia material X could be kept, but hidden.