Buses and airplanes are GREAT reasons for the nub. When you're already in really cramped space, sometimes it's elbow gymnastics to move your hand down to the touchpad, and the nub allows fairly quick/accurate mouse movements without bending your wrist into some weird contortion or elbowing the person next to you.

I fell in love with it after building a large bulk of an app on the 17 express bus over the course of a year.