Detachability.
For a quick 5 minute session, having the folded down cover in your hands is fine. But for an hour or two it's downright uncomfortable. It's even more important today with Apple's ultra-thin iPad Air 2 because as the iPads get thinner the covers get thicker as a proportion of the bulk in your hands.
A removable magnetic Smart Cover is perfect protection when you need it and a terrific stand when you need it and it disappears completely with a tug. Once you're sold on the awesomeness of the removable Smart Cover you need a complimentary back cover for protection and that's where things get tricky, there just aren't enough choices out there in black as a perfect match, hence the Khomo boxcutter surgery.
BJ