The USB Type C will be able to charge the laptop. I'm sorry but Apple should at least put two on these one on each end would be a good start or 3 two one one and one with the headphone.
Having one jack will just result in people being forced to choose between charging and using a usb key or having to purchase an expensive docking solution that's a good chunk of the device cost already.
Apple could *gasp* put them all on the same side and make a docking solution for it themselves. We do like to hook our laptops up to large external displays on occasion.
The more I think about this the more I think one could work. If this were to be the case I would ideally want:
1) The power supply to have a longish cable with a male/female USB-C connector back-to-back. This could be plugged in to the machine and would duplicate the port out the back, still leaving one.
2) A "docking port" provided in the box into which this power supply can be plugged, which has ethernet, SD card, video out of some description (say DP or HDMI or both), audio out, and three or more USB-3 ports. And (most importantly), a cable about a metre long with a duplicated USB-C male-female port.
On the desktop, this port box thing would be out of the way, with cables connected to it for your speaker, ethernet, display, and what-have-you. Just one cable leading to the machine.
When you're on the road, you take the Air and the main power cable alone, leaving the dock-port thingy behind on your desk.
To me, this would be a better and cleaner solution than having half a dozen connectors on the machine that need to be plugged and unplugged whenever you take the machine anywhere.