I can think of a couple reasons why I would want it on top.
One is, when I use my GPS I sometimes have to plug the charger in to the phone, When I do this I can't put the phone in the holder because the charger goes in the bottom and the holder has no holes in the bottom. Plugging in to the top I could charge it and set it in the holder and see and hear the phone without it sliding all over the seat.
Same with wanting to charge it when the battery is low while driving.
Listening to music etc.
I am use to the power button being at the top right corner of the phone, and would hope that is also where it is on the OB.