I agree with jmroth. I seem to like it the other way better. If a site is set to have 1 gig space, then I should be able to set users on that site as -1, and basically to me that means that user can use up as much space as they like, as long as a combination of all users is under the 1 Gig limit.
So if I have 10 users, some might use more, or some might use less, but if they all have -1, then it's flexible how much they use, as long as the total of all the users remain under the entire sites limit.
This seems easier to manage in my opinion.