I would say a Volt has as much right to plug in as a Leaf. if I had a Volt (which I considered buying) I would try to run in EV mode 99% of the time, if possible.
Incidentally, I have a PHEV Prius (aftermarket kit, not the official toyota version) and I try to drive it in EV mode as much as possible. My wife drives our Leaf, so I am still driving the Prius. I've never plugged it into public charging before since there are almost zero of those around in the Ft.Worth area. But if there were, I'd certainly take every opportunity to charge it up.
Now, granted. If it were a situation where somebody NEEDS the charge to make it home, I'd be happy to let them charge instead. Although that is probably poor planning on their part. But if they are just trying to top off the car to feel good, then I'd say my need is just as much as their need.