In many countries (notably Australia and New Zealand, but in most developing countries as well), ISPs impose usage caps. Not in terms of bandwidth (corresponding to QoS), but in terms of the total amount of data that can be downloaded or uploaded over a given period of time, usually a month.
Including the xt_quota kernel module will make it possible to use the "quota" extension to iptables. Together with cron.daily, for example, this will make it possible to set daily limits, say (perhaps of differing size for different users), ensuring that the internet cap is only reached at the end of the month and not long before.
Besides the ISP-capped scenario, it might also be useful for parents as an alternative to time-based usage limits for their children, or perhaps for housemates sharing an internet account.
At the moment I use the iptables quota extension separately on each of my hosts. This is a much less elegant solution than doing so on the router, and is open to abuse. For example, the quota counter is lost during a reboot. Router uptime is typically orders better.
Gargoyle has implemented similar functionality into its GUI. That's a lot more than I'm hoping for with this request. Simply including the kernel module will be great for a start, allowing the extension to work from the command line.
PS Thanks for the great work!