You are correct, FireWire can support true bidirectional communication between devices while USB, which is cheaper to make, Only supports data transfer in a single direction at one time.

How about a Gigabit Power over Ethernet (POE) anus warmer. You can get 48V up to 350mA for a power of 16.8W. With USB 2.0 you are limited to 5V @ 500ma for a power of 2.5W. With Gigabit POE you get a much higher bidirectional speed and you get a longer cable run.