I have a 12V plug on my motorcycle (fused, regulated. I installed it for like 10 bucks). I also have a couple of solar chargers, they hold 2 AA batteries. They take about 8 hours of sunlight to fully charge and can put an iPhone into the 70% range.
When there's sunlight, you want to be out riding, not sitting around waiting for your phone to charge! And plugging/unplugging the phone every time you get on/off the bike is annoying.
