The pass throughs that don't have a battery are drawing all their power from your USB port which supplies the 5v power. If you overload it, hopefully the overdrawn port will just shut down, but the high drain can't be good for the electronics. I'd rather have one with the battery since the drain is from the battery instead of the port itself. The USB port just charges the battery and the battery supplies the power to the E-cig.
Plus, you can use the passthrough unplugged for much longer than a standard 510 batt. The 510 bat is 150mah and the passthrough bat on mine is 360mah. I sometimes use it away from the computer when watching TV or put it in my pocket to walk around.