Keep in mind the mileage is never actually free, even if the supercharging is. For leasing the cost for a 15k mile lease vs a 12k mile lease was about 24 cents per mile (at least on the quotes I was given). The overage cost on both of those was 25 cents a mile.
This is also true for purchases, although since it doesn't come around until its time to sell the car it can be hidden and much harder to calculate.
True, it is about .16 cents a mile since it's another $40 per month at the moment, but I drive quite a bit.. at least 15k miles per year and i'd hate to have to pay for supercharging on top of the .16 per mile.