[citation][nom]JimTech[/nom]Why don't laptop vendors include a USB port on their batteries? A lot of us carry spares but can't access all that stored juice unless we power up the laptop.[/citation]
I like the concept, but it would require extra circuitry to step the voltage down. This would add cost, so I'd prefer it as an option. Besides, you could always power up the laptop and charge off the USB port. That would waste power, obviously, but it would certainly give you enough excess juice to charge your phone in a pinch.