I visited with a guy driving a 2016 Model S at the Craig Supercharger site. He had a 7 mile commute, one way. Always made it a point to stop and charge before returning home from work. Although he had a Level II charger installed at his home, the draw of "Free" kept him stopping. Early Model S included free lifetime charging. For myself, the attraction of charging at home, while sleeping or puttering around the place would probably overcome the few bucks I would save by using the free supercharging.
Guys above are worrying about bad things happening at supercharging sites but the same can happen at gas stations. Pulling into your home garage and charging behind a closed garage door should really appeal to those worried about muggers.