The suitcase works due to size limit and structure/padding in it. Bike pumps are sharp, also have a sleeping bag and tent etc all in the suitcase. It also 'restricts' what I pack - no room for extra stuff that I don't need, plus it keeps things rather neat and orderly as opposed to stuffing into a duffel bag. I also haven't come across a rugged but lightweight duffel bag that wouldn't add extra weight and take up room in the panniers.