James Hetfield (Metallica singer and rhythm guitarist) holds his pick in a 3-fingered grip in the way you describe.
If these people are faster at flatpicking than him then I say listen to them, but chances are they are not
I personally find it easiest to hold a pick with three fingers for strumming and two (pads, not side of finger) for fast flat picking. Its very quick to side you middle finger onto and off the pick. No rotation required. But I would practice what feels natural to be honest. The world is full of examples of the 'right' way to do things and poeple who have not followed the herd and done as well or better doing it their own way.