The purpose of missionary work is to aid people body and soul.
Strictly speaking, the real purpose of missionary work is to spread diseases and show dem backward natives the joys of white bread and flatulence.
Really, there isn't much you can say to defend missionaries. Whether they are Jehovah's Witness or the damn morms, they are knocking on your door because their way is The Right Way and they are going to save you from your way which, obviously, is The Wrong Way.
I deal with them by closing the door in their face. I don't call them dirty names even though they call me dirty names (that's the job of a missionary). I don't raise my voice and I don't slam the door.
I quietly close the door and forget about them.