I've met people who do it. The idea or premise behind it is that your index finger 'drives' the gun and you are pointing at what you're shooting which forces some kind of additional hand-eye coordination. My hang up with it is that if it takes any attention away from your sight picture or changes your grip/handling in any way at some point you're going to hit a wall, so to speak, in terms of accuracy or performance.
This is my take on it so don't think I'm being scientific or anything.