This obviously depends on context, but taking someone else's 3d model from partial to finished is often slower than starting over from scratch, because of topological considerations. For a random piece of environment, it's probably a good workflow, but for hero objects, where topology is downstream from the art direction, I'm not sure you get much time savings from a partially-complete model.
Of course, there may be workarounds to topological speedbumps with respect to deformation/animation, texture, detail, etc, that would make the 80% right 3d model very useful.