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.
It is a problem that (3) is something that could get you sued, depending on the specifics, although I'll grant that almost never happens.
I have work for major big brands that I suspect would land me more clients but I can't even mention the relationship. I signed the NDA, so it wasn't a surprise, but it is frustrating that the power imbalance allows that.
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.