It's just best practice. This is what the FreeBSD-Documentation has to say about that topic:

"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.