For some guns and some people, that's a comfortable and accurate hold. For others, it's not. I haven't found any good reason not to use that hold if it works for you, but I've certainly read a lot of comments from people who just say it's wrong. If you can put five rounds in a big jagged hole rapid-fire with that grip, hey, who am I to say not to use it?
So which great-grandpa gun writer bestowed that bit of wisdom upon us, so that now it's repeated as gospel? Anyone know?
On the other hand, revolvers (magnum and black powder especially) are not nice to index fingers if they're not held down and away from the cylinder/forcing cone gap! I've gotten a couple of nice burns from forgetting to tuck in that finger.