Just to clarify, I am thinking of this as an additional option among several forms of subscription.
Right now I pay month to month and don't ride indoors between April and November, so I purchase 4 months out of the year, and get bombarded with special offers for a discounted annual subscription that would result in me paying more. But if I had an annual subscription where my distance, or time, or whatever, was metered (like a utility), I would buy that, and would probably use it more.