http://www.opengroup.org/onlinepubs/7990989775/xbd/envvar.html

<quote>
SHELL
    A pathname of the user's preferred command language
    interpreter. If this interpreter does not conform to the XSI Shell
    Command Language in the XCU specification, Shell Command Language,
    utilities may behave differently from those described in this
    specification set.
</quote>

So using shells for users who prefer csh variants is a no go.