I like the pay as you go model as I really fear the impact on travel convenience that 300k M3s will have.
To minimize Supercharger congestion they could even charge per minute plugged in - not by how much electricity is being used. As many of you know Tesla's start charging fast then slow down to protect the battery as they get full. Set the pricing so Tesla only breaks even on the electricity costs at the peak charging current then once the car starts to taper down or, even worse finish charging, the profit is an incentive to move your car, that also funds further Supercharger expansion. As an example at $0.25 per minute it would cost $4 for a 20min 1/2 battery charge, $12 for a 95% 45 minute tapered charge, and $15 an hour for 'parking'. Even though the fee would seem relatively small to some, I expect it would significantly reduce Supercharger congestion.
Some posters raised the hassle of billing and collection as an issue, but I don't think is has to be. Supposedly all the Supercharging data goes back to Tesla including start and stop times. If they required you to have a credit or debit card on file to enable Supercharger access then their system should be able to bill and charge you easily every month for the time that you are plugged in.
Tesla could even be nice to road trippers and make the first 20 minutes of charging free if you are more than 100 miles from your billing address to align it with their more recent 'free long distance' wording.