For modeling "old" Terramodel is 10x better than Civil3D.
The contractor may request the Terramodel file so they can adjust areas when needed without having to send them out for minor revisions.
When a surface is imported into Terramodel that was created in another program, looses a lot of the capabilities to easily adjust the surface.
When editing a surface in TM you are not messing with the "links" between points if not part of a sloping 3d line.
See attached pics of the same surface with and without the "links". Having the "links" as true sloping lines makes the surface larger and harder to manipulate.