Sometimes only having wheels isn't enough, like traveling throug the cobal stones of Europe or the sandy roads of the African Sahara. Convertible bags are a great option because you can wheel them when the road allows, but just as easily carry themk on your back when it doesn't.
Duffels are graet bags for almost any situation, because you can just stuff them as full as you want. Their simplicity is a thing of beauty! But sometimes when your gear weighs more than you, you need a little assistance lugging that duffel bag around. Wheels are your perfect partner to get that heavy gear from point A to point B.