Imagine running a VPN hub for about ~800 customers that your IT people do remote administration for. We moved to this model because each customer tends to use their own preferred VPN agent, and multiple VPN agents installed on the same machine tend to fight each other to the death (and the host's).
To avoid ip-range clashes between customers, traffic inside the hub was IPv6-only, with each customer given their own prefix. Routing permissions were based on group membership, so we had a very easy and secure way to make sure each of our users was only able to reach their own customers, and personnel changes did not require any customer-side changes.