We've been doing a lot of talking lately around here about the use of a "perfect scene" in our development process.  Back in AOM we used a lot of conceptualized screenshots for prototyping things like god powers and our water effects, but really didn't get a perfect scene, as in, a representative high water mark for the graphics in the game, until fairly late in the process. 

One of the approaches we are taking to move that earlier in the developmental process this time is working more towards hand-drawn (or painted) representational sketches.  That is to say, concept art not of units or environments but of what a hypothetical screenshot might be.  This approach doesn't always capture the details of representation and rendering perfectly, but can really help for composition and structure.

I view the role of concept art "screenshots" as much like other forms of a concept art... a modeler needs a direction to go in that a traditional concept drawing provides, and doing that at the higher whole-rendering concept level can provide inspiration and guidance for both artists and programmers working on how the world looks.