I feel that my road bike (V-Strom) handles better with the side cases rather than the top box. I think the top case buffets. But a lot of people don't want to ride with side cases. On the V-Strom they are very very wide since they stand outboard of the upswept exhausts.
A fixed, hard, locked case is very handy for commuter use or errands. What with the price of gas, more bikes are being used that way and I think there will be more and more hard cases on the road. In the few European cities that I've personally seen, I'd say that the majority of the bikes and scooters in-town had top cases on.
I don't want to ride with my computer bag, my briefcase, and my gym bag all hanging off me. With the two side cases, I can stow all three out of the rain (around here, even if it is sunny in the morning, it mught thunderstorm in the afternoon). With only one case I could fit 2 out of three inside and let the gym bag get wet I guess.
Advanced pancreatic cancer found 04/2010. Have outlived +/- 97% of patients with this diagnosis, but 08/2013 cancer now in liver, vascular system and lungs with 20+ lung tumors. Sick/weak sometimes, not riding much. No more treatments & now under Hospice care.