There was a talk at last year's D conference how Remedy Games have used D as their "scripting" language:

http://dconf.org/talks/evans_1.html

I wonder if some of the same points would apply here. The short version of the talk is that D compiles much faster than C++, has limited C++ link compatibility (e.g. classes, but not templates), and overall has nicer syntax / language features than using C++ directly. Metaprogramming / compile-time introspection allow automatically serializing/deserializing data to allow updating data structures without restarting the engine.