Why should you include it: Mksh is more lightweight than bash or zsh, yet more functional than ash/dash. It allow portable scripts, does sizing and terminal adjusting better IMO.
It is open source, yes.
A link for to the most recent tarball: http://www.mirbsd.org/MirOS/dist/mir/mksh/mksh-R56c.tgz
Edit: a new version came out.
Edit2: a new minor version came out.