No one would argue that barbers do not exist.
God is not tangible, not definable, not explicable, and we can't meet him in any provable fashion.
So the metaphor fails simply in making the case that "God" is like a barber.
I'm not much for the "Too much evil in the world" argument, since when I look at it from the other angle ("let's assume God is evil"), the world would be much more hellish... but it's not. It has its share of pain but some good too.
So we're left either with "God doesn't exist" or "God allows evil/suffering for some reason related to the greater good."