Here is my take on it and it builds on what has been said.

MAT Files were part of a series of hacks that folks were experimenting with to get Poser to do more things with different kinds of Pose Files.  This was around Poser 4 and Poser 4 Pro Pack.

When Poser 5 came out, there was MT5 but it was single-material only, so folks continued to use MAT Pose Files because they could change materials on an entire figure (or smart prop) at one time.

Material Collections were introduced in Poser 6.  But for 2 reasons, folks continued to use MAT Files.  First, DAZ Studio could not use the Material Collections nor did it acknowledge the Materials Library.  Second, it usually takes one or two generations for the overall community to adopt somethng because most people were still using Poser 4/Pro Pack or Poser 5.  DS didn't catch up to using Poser's Material Library until version 4.

Now on whether or not I make MAT Files?  I haven't made them in several years now, thankfully.  They are inferior to Material Collections, which work on Standalone Props - MAT Poses do not (unless you hack them a certain way and a Figure is in the scene).  Like you Shane, I believe Poses and Morphs go in the Pose folder.  We need a Morphs folder as well.  There is no good reason these days, post-Poser 6 and DS 4+ to continue to use them.  That is my general thoughts and I won't be making them.

I did write a tool in the MP to convert the MATs to Material Collections easily because, like you I was seriously tired of dealing with them.

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