Yeah, but the second port would not be full USB 3.1 due to the core M chipset, nor support all the versatility of a true USB-C port such as HDMI, mDP, etc. In addition, the logic board would have to be enlarged to accomodate the controller for the second, crippled port, creating possible thermal issues in the chassis and necessitating a smaller battery capacity. This assuming the physical size of the machine were maintained. This has already been covered in this thread and several others.I totally agree with you. Despite being able to afford the rMacbook as a secondary machine, i am not willing to pay a ridiculous amount of money for basic adapters. I need HDMI and occasionally VGA for presentations, which means 2x89€ = 200$. With a second port, this issues could at least be reduced, because i could buy a third party adapter and i would still be able to charge the machine with the other port. Besides the lack of proper 4k support, this is really a dealbreaker for me. Also i am not sure, where most people get the confidence that mechanical wear and tear won't be an issue in the future. If the single port for charging/connectivity (without Magsafe) breaks, i would face high repair costs or i could throw away the whole machine.
Skylake core M will make two full 3.1 ports possible, but space and other consequences of having to put in a second controller for the port will remain. Thus I believe that unless Apple finds other places to gain back battery efficiency to maintain the same life or better, such as the display technology, then we won't see a second port in the 12" rMB. A 14" model on the other hand, would probably be much easier to add a second USB-C to, although I still think that that charging would be limited to one of them. If that were to happen, it would be the end of the MBA for sure.