I would think it would be a lot more useful if you could charge it from your phone itself. Pebble's battery is 130 mAh[1], so (unless I'm mistaken) you could charge it using 9% of an iPhone 5s' 1560 mAh battery[2]. If the charger had USB passthrough you could charge both at once.

Of course, then you'd be carrying an extra cable, but that might fit with those in Pebble's target demographic (in a bag along with your tablet/laptop and such)

[1]: http://appadvice.com/appnn/2013/03/the-pebble-watch-is-impos...

[2]: http://www.ifixit.com/Teardown/iPhone+5s+Teardown/17383/