Due to the limitations of the USB format, it is not ideal to charge the gamepad from USB. The battery in the gamepad is 1500mah (http://www.imgmr.com/index.php/Wii-U-Ne ... ailed.html
and USB can only charge at a rate of 500ma - which means it would take 3 hours (if there were no inefficiencies, but there are, so you're really looking at more like 4 hours). What this means is - since the gamepad goes through it's battery in 3-4 hours, the USB doesn't provide enough energy to play on an empty battery- you would drain it faster than it could charge.
Now, if you are planning on sitting down for a long time to play and plug it in at the beginning, it could keep you going significantly longer, but then why not just use the AC adapter?