Kyelectric, the problem is not what you know will be plugged in, it is what might be plugged in.
Take the two ungrounded conductors on LEG #1 and LEG #2 and load them to 20 amps each, the neutral will carry 0 amps.
Now take and add another conductor to LEG #1 and load it to 20 amps.
All three conductors are now loaded to 20 amps.
One conductor on LEG #1 will cancel with the conductor on LEG #2 which will leave the neutral carrying 20 amps. (the current on the other conductor connected to LEG #1)
Now, if the cicuit feeding the conductor on LEG #2 is turned off you no longer have a MULTI-WIRE BRANCH CIRCUIT, and the grounded conductor will no longer be a neutral, it will simply be a grounded conductor and will be carrying the total of the two ungrounded conductors on LEG #1, which is 40 amps and your # 12 would be overloaded.
Roger
