That's only my opinion...
..most of the admins 'use' bandwidth so that users will know they can only use certain amount of transfers...but for what? - so that connection speed for next users won't be that much slowed down...
imagine to have 50 users that will not use account for some amount of time, because they are busy [ whatever..], and bandwidth goes to next month....however, other 50 people is using account "normally" and in next month ...- users will wait for page to load few minutes
so I don't think it would be good feature - unless it can be easily turned off ;-)