To hell with whether or not its a deterent, will it get dangerous criminals off the street for good so that the rest of the population can sleep a little better at night? If so then, I say... lock 'em up and/or shoot 'em.
I once heard the attorney general of MO say this in regards to the death penalty: "You don't kill a mad dog as a warning to other mad dogs. You do it so they can't hurt anyone else."
You're exactly right. Banishment, forcing a criminal to leave a society, is the second oldest form of punishment known to civilization.