This is why I believe that it should be a metered pay-up-front system. Let's say Supercharger access costs 2k upfront, and you get $1500.00 kwh of Electricity at, let's say $.16/kwH. Let's say the car gets about 4 miles/kWh. Then you'd essentially be getting 37,500 miles to drive before you have to reload your account. That would mean that most people would never reload their accounts, and it would essentially be "free" every time you go to a super charger, for the life of your ownership. People like that mail carrier in the TeslaMotors video who drive around rural roads all day everyday might end up using that in two or three years, and thus, would have to purchase another $500.00 or $1000.00 pack to reload their accounts.
Or, maybe you could automatically start off with $250.00 applied to your MyTesla account with the purchase of the car. The electricity itself would be sold at a locked in rate, determined by Tesla at the time of purchase. They could then incentivize you to purchase larger packs (ie a $250.00 pack for $0.20/kwh, $1000.00 pack for $0.15/kwh). At that rate, everyone would start off with 5000 miles of supercharger use but could add to it at any time. To your point, someone who practically never uses superchargers would never pay extra for supercharging. For those of us who LOVE road trips, we'd happily pay the $1000.00 to drive an additional 25k miles... or even just pay the $250.00 to get an additional 5,000 miles.
I haven't worked out the exact pricing structure or anything, but it seems that knowing there is a cost associated with Superchargers should be enough to dissuade people from camping at them when they don't need to, as well as begin to recoup a portion of the money associated with the service/infrastructure.