That's different. Storage is 'in use' when you're using it, period. But when you have a dropbox subscription and you don't use your storage then that should not be charged imo.
It might be better to separate the two use cases as: "data at rest" and "data in use" like Amazon does with S3 where they charge separately for storage and transfer costs. If you do not access your data stored on S3 throughout a billing period, your invoice will only reflect those costs -- your usage costs will be stated as $0 for that month.
