I think it was a problem with the language as well as how they handle it internally. It was basically the algorithm that dictates how the language works, and consequently there was no way to have one material touch more than one other material. But I might misremember.
Anyway, I'm looking at this from the user's perspective. I wanted to do some physics-based ray-tracing with lenses and pbrt is what I ended up trying. As such, I really needed the multi-material aspect to work correctly. Also, it would be nice to be able to describe surfaces using a z=f(x,y) kind of formulation, or a way to place a hook in the renderer.
