I am not animating or rendering realtime, so I can use higher detail models so I have less dependence on a a displacement map to give me a more accurate silhouette and form and so can use normal maps to give me just the higher level details such as skin pores and wrinkles which would be stupid to try to achieve with micro-polygon displacement alone in my opinion. This method is just faster and simpler to implement. Just whack in the normal map and render.
Although I could export the displacement as well as an option.
Normal maps are just an alternative to bump maps, with nicer results due to the extra info in them (colour channels) and are not just limited to game engines but are useful for any 3d rendering and can be used in mental ray although some of the mia shaders can be a little buggy sometimes.

Cheers

-Gio