Earlier quoted context omitted.
I guess I was more curious about the cost/benefit. Given a well defined standard like POSIX, and the difficulty of swapping in a new shell into Windows, I wasn't seeing why you'd buck the well established trend.
On windows a command interpreter is just a console application like any other console application, you just run and use it, no "swapping" involved.
xterm -e /usr/games/nethack
xterm -e /bin/sh