As others have mentioned, it's a hold over from the way things used to have to be done. As I recall, in P4 there was no Materials Library. I don't have anything before P8 installed anymore to double check, but I'm pretty sure that the materials library, along with the MC5 (now MC6) materials file type, were introduced with P5.

Prior to P5 (I'm sure someone will kindly correct me if my memory has clouded a bit), the only way to change materials on a poser figure was through a "matpose" file, which "needed" to reside in the Pose Library. So, any vendors that are keeping P4 compatibility in their products will have matpose files in the Pose Library (many also provide have MC6 files in the Materials Library as well).

Edit: Netherworks beat me to the post - and has better info as well!!!

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