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Procedural textures can have parameters, so why not materials?
Why? Possibly an oversight, possibly choice.
As things are at the moment, you wouldn't be able to do much with a parameterized material. Much of the power of texture parameters is that they can be mapped to parts of an object mesh. There is no way to specify points inside an object. (There have been a few discussions about some kind of tetrahedron-based volume mesh, and I wonder if eventually the voxel tools might support internal-point parameters... but not yet.)
You would basically be limited to per-object parameters, that would vary in time. (Not saying that that wouldn't be useful, but some of the same thing can be accomplished inside the procedural editor.)
Per-object parameters are fine. All I want is to be able to create a material called, say, "glowy plasma", and pass in parameters defining its color, as opposed to having to duplicate the material for each color I want it to be, and editing all the copies when I want to make a change.
Closing as duplicate of https://sourceforge.net/p/aoi/feature-requests/266/
Further discussion should go in that Feature request.