For dirt jumping I never switch gears, it is all about the flow and momentum. I will also be using it at the local BMX track witch I also stay in one gear. Same goes for when we ride street.

It is a large hub because it nees to handle the forces placed on it from the disc brake and also the pounding it will take when jumping (if there is no tranny).

Also, unlike most single speeds it uses a freewheel just like a mtb hub so you do not need a separate freewheel, only a sproket which will give you a broader range of ratios to use depending on what you are doing.