I have a convenience project that sets up several others repositories for ease of development and the things you call rare/a bad idea are perfect for this use case. It's working very well so far.
For that use case, how often do you want the other repositories to be fixed to some particular hash, however? At least every time I've wanted such multi-clone behaviour I've wanted to have all the repositories on HEAD of some branch, and very rarely some fixed commit.
During development I don't really care, but every once in a while I commit them so that there's a known-good configuration to start over with.
I do generally want a fixed commit. I often want to improve the code in the submodule possibly (voluntarily or accidently) introducing breaking changes without worrying about breaking repos that contain it as submodule.