Maya (for example) cant handle all the polygon that vue if you import them you lose a lot of the control of the animation if you import the objects into vue.

We did a fly over a land scape. built the ground and buildings in maya and populated the battle field with some trees and bushes and then cover the hills with trees. There was 128 million polys within vue and we were able to take that scene into maya to animate and render. The rendering is also quit cool because it mixes mental ray with the vue render automatically - which saves a lot of time