IIRC the difference between a .pz2 that changes material settings and an .mc6 that changes material settings is 2 lines at the top of the file. In all other respects they are identical. In fact you can switch them back and forth by changing those two lines and the extension. 

 

As for why they are in Poses? Tradition. Poser only had one place to put them originally, Poses, and that is still where most people look for them.