I have two sticks and have both of them plugged into the USB port. No problems at all.
Really the only reason I would see using the wall outlet with the cube for power would be, if the TV does have enough power coming out of the USB port to drive the ROKU.
That's why the ROKU has the extremely short USB cable.
The only time I would use the cube and the wall outlet is if i were having weird problems with the Roku, it shutting down, other bizarre issues while the TV is on.
As I stated in an earlier post, the USB cable is only for power and the HDMI connector is only for the interface.
As the previous poster suggests, maybe it is time to upgrade your TV..