I sometimes wonder about the true reason for ERDTT.
Granted, we live in a world of tradeoffs. Maybe, to meet the required levels of defrost at cold ambient temperatures would have required an even larger electric heater in the Volt, and that may have lead to increased cost and lower EPA efficiency ratings. On those cold days, the heater would have drawn so much power that the Volt would have reduced power output (the battery can only supply so much power). People would not be happy if they had less performance on cold days.
In addition, the battery pack performs best in a temperature window of about 35 F to 78 F, IIRC. As you get outside these temperatures, you must heat or cool the battery pack. This is yet another power draw that the Volt must contend with on cold days.
My personal belief is that the GM engineers weighed all the design parameters and came up with a good compromise. The Volt is an EV is most driving scenarios, but in extreme cold temperatures, they utilize the ICE to provide cabin heat and battery pack heat. Since you have an ICE anyway, this was the best way to solve the cold temperature issues.