Ashley - I totally understand what you are trying to say, it would be bad for your company if there would be a standard one-time payment option. But please, my point might also be a good idea.
Imagine excluding the Free Version, there would be a onetime payment pricing... That feature would be only for learning the program but once you exported a game, you can't export anymore then you'd have to go upgrade to a Personal Subscription.
This would be a good idea for hobbyists if you think about it... Hobbyists can't afford it anyway so it's best to just to do this and accept their money to learn as long as the preview option is available and when they are finally ready to make a professional game, they can go and subscribe to the subscription that suits their needs.
Besides, even if they don't pay anymore, they still can't export anyway and if they make a new account, they still have to pay an additional $60.00...
So it's a win for both Scirra and User.
Ashley & Tom . It might be worth considering.
Or if you don't like that maybe a pay per export may also be an option.