I just bought one of these (front light) on the strength of the USB plug-in charger.

I'm surprised by the comments detracting from the product. I think it's an ingenious idea. Also, I don't think the light is targeted toward people going out for night rides and extended rides in the dark.
It hit me as a perfect light for commuting to/from work.

Plug it into your computer all day and have a full charge for the ride home. Easy to switch from one bike to another if you need to.

It might not hold a charge for long but probably long enough to make it home within 2 hours. It's not a full-on night riding lamp.
It makes you visible and street legal at night.

As a bike commuter that sits at several computers all day it made total sense. Your average computer user probably isn't running into an issue of filling up their ports.