I know 50MPG isn't that great but that's exactly what the Volt gets... So if the Volt gets 50MPG and the Prius gets 50MPG, why would you get the Volt unless charging from a wall outlet so that you can get "40 miles" of pure electric driving is something you desperately want. When I drove the Prius for the time that I could, I actually didn't get 50MPG but actually 60 on the highway and 80 in the city. If you want a vehicle that gets really good mileage, wait for the Aptera Gas Electric Hybrid which will get 130MPG on gasoline alone.. (This means the battery is fully depleted)
		
		
	 
The commute is one thing, but what about days you aren't commuting? Cars  get their worst mileage on a cold start, short trip. Being able to run errands to the grocery store, post office, around town.. without having to worry about fuel would be nice.
Also, what if your commute is 50 miles? If you can't plug in at work, you'll end up using some fuel. But, for example, 250MPG @ 40 miles and 10 miles at 50MPG is still 210MPG overall. How and what are they going to rate it at? On one hand, rating it in the familiar MPG makes sense, but how do you even calculate that? Watt hours per mile would make more sense. 
If your commute was 120 miles round trip and you got 250MPG for the first 40 miles, 50MPG for the remaining 80 miles, you would average 133MPG.
Those are pretty impressive numbers, overall. Also, how many miles the charge actually lasts will depend greatly on driving habits, terrain and weather conditions. It might only be 30, but it might be 50MPC(miles per charge? lol). Also, let's not forget about regenerative braking. It is not difficult to encounter situations where you can recharge your battery quite easily. I think that's something most people are overlooking. Regenerative braking can recapture a lot of energy.
A 300V, 50Ah pack would take approximately 30kWh to recharge from empty. That's about $3.50 worth of electricity at the national average of ~11 cents per kWh. It would never be completely dead, though.