The ASS isn't reasonable in everyday driving unless you live somewhere you might not move in traffic for long periods of time. It's the CAFE test that simulates situations like that so one way to inflate the fleet standards is to do something that mimics saving gas during that situation.
As others have said, in very short duration shut downs, it's hard on bearings and is hard on the battery (although, with current technology, you can usually restart within 240 degrees of crankshaft rotation). All in all, another feel-good function brought to us by bureaucrats with good intentions who also are idiots.