I think it's more about posing the models and stuff. The facial expressions are sometimes pretty blatant, but often quite subtle which is cool, and would take some time.
I don't play enough of these games to see recurring characters, but I imagine it would be pretty jarring. I think the doctor from CoBD is the same base model as Jill, but there's been enough tweaks to differentiate them.
Personally I don't mind any of the characters, flaws and all. People get a little bit carried away (although I know a lot of it is in jest) at things like Maya's goblin features, Jill's alien smile and all that, but it's just a bit of fun. I'd hate it if all the characters were completely flawless models who look great in any situation (they are mostly already a bunch of stunning girls).
To me, the perceived personalities (which is weird, I think it must be something we subconsciously attribute to them) make them more enjoyable than purely their looks. So spending an eternity to get the models perfect wouldn't enhance the game much more.