Having just completed my first real Zbrush job, modeling and texturing into a Vray pipeline, I hope I’m not restating the obvious, but here goes:
If you put more than one material on an object, each material has its own values for diffuse, reflectivity, glossiness, etc. All the eventual Vray shaders needed diffuse and reflectivity maps, at a minimum. As it was, I had to either paint separate maps and/or manipulate maps in Photoshop. It would be great to be able to bake the Zbrush material values into separate attribute maps.
Another very common production tool is to output multimattes, where pure red would be assigned to one set of items, green to another, blue to another, to aid compositing. An alternate workflow from above would go as follows: if separate Zbrush materials could be output as rgb multimattes, those maps could be used to build layered shaders in the rendering application.
One other aspect of the project was we were working in a strictly linear floating point pipeline. This meant that all textures needed to be linearized in Nuke before they were hooked up into the shaders. It would save a step to be able to output a linear map directly from Zbrush.