I disagree that it is the worst argument in the world, or even that it is a bad argument in most cases.
There is something to be said for the simplicity of rules we adopt. If we say, "X is never allowed", then it is less likely that someone who wants to do X for their twisted purposes will be able to get away with it, than if we said, "X is not allowed, except when it does more good than harm."
There is something to be said for the simplicity of rules we adopt. If we say, "X is never allowed", then it is less likely that someone who wants to do X for their twisted purposes will be able to get away with it, than if we said, "X is not allowed, except when it does more good than harm."