From a traditional bullseye marksmanship style, there's nothing wrong with gripping the magwell or using it as a handstop. You're shooting long distance from unstable positions, with no real emphasis on speed of engagement.
From a dynamic carbine shooting perspective, it's a distinct disadvantage. you cannot control recoil as well for fast follow up shots and you can't drive the gun from target to target as fast or stop on the next target with as much precision. It's a matter of physics and the fulcrum effect that GTifosi described.
To convince yourself, do a VTAC 2X2X2 or 1/2 & 1/2 drill using a magwell grip on the clock. Then do one using a grip that extends the off hand as far forward and as high up on the forend as possible, using it to pull the rifle into the pocket. You will find that not only are your times faster, but your hits will be more centered with the high forward grip. You simply control the muzzle of the gun better.

