If you use the Maya FBX Plug-in to export to MotionBuilder, make sure that you use the option for the current FBX version for your export. If you do not do this, NURBS are lost and mesh normals may be inverted.

In the FBX File Format, version FBX 2011 (under the Advanced Options of the FBX Export for Maya), normals in normal per polygon/vertex mode are transformed to normals per vertex and hard edges become smooth edges.

See the Maya FBX Plug-ins documentation for details on each export version.