Monthly = Consistent money
One-time payment = Good at first, and slowly deteriorates.
Having monthly payments ensures that developers and script writers will have a consistent flow of income, however, this doesn't benefit the customers which is true. I think that scriptwriters should have the freedom like you've said to choose whether or not their scripts re-occur on a monthly basis or not even though most would be against it considering the economic benefit that is being sown by monthly payments. As far as a flax spinner is concerned, I am sure that a local script will be released for basic scripts such as that.