It isn't so much about using the battery, as it is about using the ICE/generator set in the car - in other words, you let the car sit there and cycle the engine and provide power to the house, like a generator on wheels...
I did that during the last outage, but with a hybrid car (not a plug-in). It just stood there, cycling the engine, as the power was consumed. It did that for several days in a row with no ill effects. The battery level was maintained within about a 20% window so not worse than actually driving the car in terms of wear.
The "missing link" is some way to convert the 300V - 400V main battery power directly to 115VAC, rather than going via the 12V system like I did, which is much more limited!
The reason for choosing PHEV rather than Hybrid for this, is simply to get longer cycles between the recharges, and of course the normal benefits of a PHEV when it is being used as a normal car.