Slingshot Just Broadband is 25GB, if you buy an extra 10GB data block then only use 5 of that before the month ends, you get to use the remaining 5 in the next month after you go over 25GB meaning you don't have to buy another data block yet and you get to use all of the amount you paid for.
The problem you mention would only happen if the user stayed under plans included 25GB amount for consecutive months until the data block "expired".
Slingshot data block's last for 12 months, so Aviz's situation would pretty much never happen with Slingshot's system.
Personally I'm hoping for advances in per user bandwidth shaping, then we can get rid of this artifical per GB or MB system.
x active users on the network, y total local bandwidth, z total international bandwidth.
Ideally each active user gets upto an equal share of what's available to do what they like with without shaping individual protocols.
Anagran's system sounds interesting, being used in a few US universities now. Wonder how expensive it is, probably out of the price range of NZ ISP's..