Re: Couldn't you just charge your phones overnight, too?
No. Internet access in San Francisco is essentially a fraud. Cellular coverage is made out of failure and the tears of little girls. Your cell phones have to expend 3x the power here as back home in Edmonton just to hit the tower, and you have to push a steady stream of virgins into the nearest lava flow in order to eek out a few measly megabytes of data from the telecommunications companies that extort "the most powerful nation on earth."
Even swapping SIMs from device to device, the power cost of keeping the cell tower reachable, of popping up a MiFi point so your netbook/tablet/whatever can be tethered and then using the GPS to navigate around the tentacle monster that is the bay area travel infrastructure you are going to flatten those batteries right quick.
Native San Franciscans might have adapted to the fact that this entire city wavers betwen "designed with malicious intent" and "designed by an autistic child with crayons," but visitors simply need to get from A to B while Getting Shit Done. That means relying on those smartphones in such a manner that - at best - you get 4-6 hours out of the little buggers, and that's having tried more models of the things than you can shake a stick at.
I fear charging my cell phones are an absolutely nessecary part not only of "getting shit done," but "finding my hotel afterwards" and even "not dying a horrible, gristly death by knowing somewhat in advance what the hell lane I am supposed to be in."
That, of course, is if and when the maps application is doing it's job and telling me to "turn right" directly off an onramp. But that's a rant for later…