i 've read that too, but i don't necessarily believe it. on my road bike (ultegra calipers) i can definitely stop faster from the hoods when i engage both front and rear as opposed to just the front (from the drops, i can get better leverage on the brake levers and it makes less difference). simple physics tells you that the front brake does the lion's share of the braking work, but the rear brake is not useless in my experience of decades of cycling.