Apparently, the avg cyclist in the city pedals at 50W to move forward. Now, imagine you'd have to power your USB charger at 20W at an 80% efficiency factor (which is what current high-end hub dynamos manage regarding efficiency):

Your avg city biker will only have left half of his pedal power to move his bike across town.

I recently came across a quite new rim dynamo [1] which seems to have bearings that last for ~15.000km which might be a real alternative to hub dynamos regarding efficiency. Never had the chance to try one so far since my hub dynamo is still going strong.

[1] http://www.velogical-engineering.com/velogical-felgendynamo-...