sorry, that's nonsense. Again, _every_ system will break and not boot up when you mess around with the system shell too much. Try replacing the bourne shell by a cshell on a system-V init system. How should maemo be any different in this regard? Usually the system shell must be Posix and/or Bourne, for Maemo it must be busybox in the variant that originally shipped witj it.
Because you can? You can't. You MUST NOT [RFC2119] (modulo BB-P is supposed to be 100% compatible to BB-basic, for exactly this reason to not break stuff that depends on it. So BB-P is supposed to be 'safe' for system shell, for now - until some bug in cmd syntax wants to get fixed).

I'd say e.g. FHS and in the end ultimately the system and how it ships defines what the system shell (and toolbox) is supposed to be and act alike. E.g- for mc it's perfectly sane when the package for the distro relies on diff (and depends on it) and expect it in the location it's found at when installing the default distro pkg for diff (as a dependency of mc). You could even argue that diff has to be in a certain location when FHS specifies so (it doesn't, for diff. see http://www.pathname.com/fhs/pub/fhs-...#REQUIREMENTS2 ff). Any _bug_ in mc would be in the maemo package and introduced by the pkg maintainer who failed to adapt the dependency while preparing the maemo mc pkg.