iirc, bash must be installed everywhere else and is not as readily available except on Linux.

"It is recommended that the root user's default shell remain unchanged since shells which are not included in the base distribution are installed to /usr/local/bin. In the event of a problem, the file system where /usr/local/bin is located may not be mounted. In this case, root would not have access to its default shell, preventing root from logging in and fixing the problem."

I guess there is nothing wrong with changing the default root shell to /bin/sh, but I don't know why anyone would like to do that.