I honestly can't even come up with a valid reason for something like nested venvs. At that point it seems more like someone's doing something wrong than 'things get tricky'.
If you have unlimited disk space, sure. Otherwise, it would be really nice to be able to layer venvs so you don't need to have multiple copies of a package installed.
Isn't that basically docker images.
My virtualenv for work comes in at 847Mb (actually a lot more than I expected). I have a 2Tb disk attached to my machine. For all intents and purposes, we do have unlimited disk space.
If anything it just simply seems like a dumb idea to do so. Virtualenv is simple and works really nicely. Why anyone would want to add unnecessary complexity is beyond me