currently it's best done in post beacuse you have no other option
Lets say you are working on a low budget medical presentation and you wish to hide body parts then hide parts of the medical device and then animate it with transparency.
(Of course, everything is done from several angles.)
Currently you will have to render 3 passes with possibly some masks for each shot, then compose them. Shadows and reflections are an additional headache.
OR, you can animate the transparency straight in 3d and just render