MAT poses for P4 were originally a hack, someting that Poser wasn't designed to do.
Poser 5 introduced the material room, where you could change one material at a time. Applying a collection of materials, however, still required a MAT pose.
Poser 6 introduced material collections which took over the functionality of MAT poses. MAT poses still work in Poser 6 and later, and I don't expect that the Poser development team will drop that functionality anytime soon - it would mean restricting the kinds of things that a pose file can do.
Actually, Poser 6 material collections use the same structure as MAT pose files, and a very simple edit (replace the word "mtlcollection" by "figure" and save as .pz2 file) will turn a material collection into a MAT pose file. The other way around should work too (never tried). 

It is not just about the procedural shaders, it's about the one-click solution to load a material or a collection of materials.

Should vendors drop the one-click solutions for Poser 4/Pro Pack/ 5 users?
I think it's time to move on, so, yes. Supporting a 10 year old application isn't needed. Supporting a 5 year old application, I'd say it's optional.

There's another thing: between Poser 5 and 6 the material room has been revamped, specifically, the math behind the nodes has been changed. So a Poser 5 material will behave differently in Poser 6. It is quite difficult to make matching P6 materials for existing P5 materials, and vice versa, while it seems that the math behind the nodes has remained the same from P6 until Poser Pro (not sure about that).

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