Hi UI3,
You would want a single panel so that you have a single link (single cost) of monitoring the system at a monitoring station (so you pay $25/month for your system with 10 areas instead of $25/month per system with 1 area = $250 per month for 10 systems).
Another advantage of a single panel is that it is a single connection to add/delete/program cards/codes (Users). In this case you can link the rights of one Partition (or arming/disarming area) to be conditional on the condition of another partition (for example, to have a common lobby area that will only arm (automatically) when the 20 tenants are armed. Adding a User with rights to multiple doors and partitions is simply the choice of the appropriate access level.
Another example of a requirement for a massive amount of areas would be a mini-storage facility with 100 lockers. In this situation, you could have a single panel and only 2 proximity readers ("in" reader on the gate and "out" reader on the gate, for example). Each storage locker would a separate partition with separate rights/separate calling lists and alarm dispatch instructions. When a User presents his card to open the gate to enter, he disarms his personal partition at the same time. when he leaves and badges his card to open the gate, the re-arms his partition (so no possible false alarms and impossible to forget to arm or disarm). Our largest customer in number of partitions is a storage facility with 3000 lockers (for Protege GX controllers linked together over IP with 750 zones per panel).
I could go on and on with examples (condominium buildings, large shopping centres, small/medium/large strip-malls, multi-tenant office towers, large manufacturing facilities, universities, K-12 schools...).