As Marcus_Ti pointed out, small-small is helpful if you use a variety of cassette sizes, all within the stated wrap capacity of your RD. I am currently running a triple crank with a 12-27 cassette and a short cage rear derailleur, so small small might work if I used the middle ring, but not the granny. I do big-big, as I am not going to put on a larger cassette, and want the shortest chain that won't go into the spokes if I accidentally go into big big.