If you examine it, it doesn't make sense intellectually, or in practice.
1. If point shooting is appropriate, the guns slide or barrel is what is used to point. You do not need to have your trigger finger along the gun to point it. The finger curled around the trigger does a fine enough job of pointing it because enough of the finger is pointed forward. The rest of the gun, be it the slide or the barrel (depending on the type of gun) becomes one large pointing finger as controlled by the hand.
2. When pullng the trigger with the middle finger, you do not have nearly as solid a grip or control of the gun since you only have two fingers on the grip. This is important in rapid firing and handling guns that have *any* type of recoil.
3. Using the middle finger is not ergonomic or natural while using the trigger finger is.
4. Keeping your pointing finger along the frame and pulling the trigger with your middle finger means that you must draw the gun with only your last two fingers. This is an extremely weak and fumble prone way to draw.
5. Holding the gun with two fingers curled around the grip is an extremely weak way to hold the trigger and to control recoil for follow-up shots.
6. If you need to hold someone at gunpoint, holding a gun with two fingers around the grip is a very weak way to do it.
7. Holding the gun with two fingers of the dominant hand on the grip does not integrate with a two handed hold.