Main purpose of the design?
I think the main purpose is to make things smaller and more like a complete product. So we choose to supply power from the back, not occupying the GPIO pins. In one hand, it saves lot of space. In the other hand, I don't need to do soldering. If we choose a piHat solution, I still need to solder a GPIO header if I just have a zero (without h).
Friends asks me what I am doing with these circuit broads. I think it is interesting but hard to explain what it is. In fact, I would like to share these funny creations but people with little knowledge about hardware won't understand. Therefore, I think the better way to share this to turn it into a "useful" product, like a game boy, a smart calendar, a digital camera, etc... You can say hey, it's smart calendar, if you like, you can program it into a robot cat! So that's why we are developing different caps so that you can make a product by yourself in one miniute.
Choosing the backside solution means it's much harder to send data to pi zero. But graceful shutdown is still important for some pi project. Data transition means we still cannot give up GPIO pins. Although pi zero seems to have SPI bottom pads, they are to small to use. Also taking the USB data pads means we need to have a USB hub, which might not be necessary. Therefore we are still looking for a good approach to reach GPIO. I believe there will be a best way to balance the size and data.
Why not Qi?
I think Qi wireless charging is very useful if you want to stick the pi out of your window glass. The pi can be charge through the glass. No need to drill holes. However for general charging, there is little difference between wireless charging and using usb cable. So I think you can use a Qi tag in some specific scenarios. But it's not necessary in most of the time.
How about solar panel?
The idea about running a pi completely off grid makes me excited. However we need a pretty big solar panel to supply sufficient power to the pi. It's almost impossible to compress it a small dimension like pi zero.(or maybe in some day it will be possible) Therefore, like Qi, I think you can have a external solar charge in some specific applications.