That's why I got it actually. For insurance in case a SC was full, etc. But the convenience factor caught me by surprise, and I ended up using it more than I thought I would. I like using SC on road trips, and then using EA while I'm at my destination, since EA tends to be scattered around in the city, typically at places I'm going to anyways, like Valley Fair Mall, etc.
But the cost savings was real. I drove from Seattle to the Bay area in late spring and paid $54 using almost exclusively EA. I made the same drive a few weeks ago, but only using superchargers, and it cost me $92...