Best reason I can think of for Duct tape is to repair tire gashes. I keep a bit of Duct tape wrapped around every one of my different bike pumps just for this reason... Oh sure the old dollar bill trick works... if you remember to carry a dollar bill... what if all you have is twenties, from the last ATM visit?

A couple of layers of duct tape inside the tire at the location of any serious cut can help you limp home a pretty good distance. That fully justifies Duct Tape on my bike.

WD-40 is for the chain of that good looking lady that you happen to see squeaking down the same road in the same direction as you. Otherwise, it really isn't that great a lubricant.

A good multi-tool and a patch kit will round out the list nicely.