Cash inflow-wise; your suggestion will not benefit SE economically.
So far I know many that only susbscribe for 1 month every major patch and to wrap up the story in a week or two max, Then stop thier subscription till a new patch is released.

The weekly 450-capped tomes system is added for the same reason.. to keep you paying and playing to gather them, so you won't max your gears in a 3 days.

If SE will consider a subscription of 1 week, definatly it won't be $14.99 / 4 weeks, but something like $8 or $9. Afterall SE needs revenue, and they know if you want to play you will be paying.