Agreed. Added to the wishlist.
Ideally, we should allow to use lists in all Mat ID checkings, not only at Clipping Spline.
Anyway, there are problems to keep compatiblity with existing scenes, which we must check.
Because actually the Mat.ID is an integer value, which can be exported and connected to a Numeric parameter. I'm not sure how to convert them to a text list without breaking those links.
Thanks for your suggestion.