I don't think it would be worth the additional cost. Maybe some people are always near windows and don't mind pulling their phone out of their pocket/bag/whatever to let it get sun, but it would just be an inconvenience to me. Plugging my iphone in at night keeps it plenty charged without worrying all day whether or not it's getting enough light.
Plus, I just think the additional power will be very slight and barely noticeable when you consider the energy consumption of the device. I was looking at buying a new Citizen watch which is powered by light, and any light instead of just sunlight. But I changed my mind when a friend said his Citizen often slows down and he needs to reset it due to lack of power. So, if a simple, very low energy consumption device like a watch, which is always exposed to light, can't even get enough juice from light to keep it running, then the iPhone will get almost nothing in comparison to it's consumption.