I'm in the group that uses panniers. They are slightly bigger than required, but I use my rear touring panniers to commute. My commute is 24 miles one way, so I wear cycling clothes on the ride. My panniers contain clothes for work, lunch, spare cycling clothes for the trip home, rain gear and shower necessities, including a towel. A small bag just wouldn't do, and wearing my work clothes isn't practical for a commute of this distance. So, I guess it all depends on what you feel you need to carry.