Well, it's not that black and white. The problem comes in if the rear wheel is locked, and is also not in line with the front wheel, and then regains traction quickly when the brake is released (while it is still not in line). In that case the immediate traction can flip the bike to the side and in the worst case, highside the rider.
But if the rear wheel is back relatively in line with the front wheel, letting off the rear brake may do nothing more than start to turn and the rider can go on their way. The blanket guidance of the MSF to hold it on until you come to a complete stop isn't a bad idea, but at some point people gain the feel required to allow the rear wheel to get rolling again without having to bring the bike completely to a stand-still.
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