I'm commenting on this super late, but whatever...
The reason that MS used a custom connector and not a USB port for charge is for speed. I didn't check, but if StevenEdmundG's numbers are right, MS's power supply delivers 24 watts, and a USB port can only deliver 15 watts. MS is using 12 volts but only
2 amps of current. Having a higher voltage allows for lower current, but can still deliver equal (or in this case, more) power.
I highly doubt any component within the Surface RT needs 12 volts to operate, as it's basically just a large smartphone. I'd venture a guess that the battery is of the 3.6v variety.
I know first hand that the charger MS has built can fully charge my Surface LONG before my cellphone can fully charge, and my cellphone has a smaller battery. It may be inconvenient in an isolated situation, like an airplane or long car trip, but it also
severely reduces time connected to the wall (which is the bane of portable electronics). Plus, how often do you spend more than 8 hours in a plane???