Comment 4 for bug 487456

bash is Essential: yes, and therefore grub is actually *forbidden* from depending on it. (A pre-depends would be even more wrong.)

You're free to change this as you choose for embedded distributions, but as it stands this is not a bug in the grub2 packages. If the Debian grub2 folks want to use bash features, they're free to do so. For embedded distributions I would say that it's unusual to ship maintainer scripts in the image in the first place, so you could always configure everything on a more capable system and then rip bash out at the end.