That's interesting! I do the same thing. If somebody develops a limp in my service, they can at least be assured I'll find a good place for them marching around and around in the mud. It's the least I can do to reward loyalty!
Err, wait . . .
Seriously though, when I get a good chance, I'll "screen out" the badly afflicted and let them do other tasks, such as rear garrison and patrolling. They still serve a purpose that way and every once in awhile have actually come in handy as emergency reinforcements and such.