The principal benefits of a distributed topology are local survivability and less voice traffic on the network. The trade-off is a slightly higher total cost of ownership.
The distributed model is more appropriate for your organization when a minimum voice traffic on WAN and local survivability is important. You can add an extra application server for redundancy at locations where local survivability is critical.
With the distributed model, if an application server at a remote location cannot reach the storage server, the caller experience at that location is the same as if the storage server was reachable. Call answering continues to function and users can still access their most recent messages. For more information about how Messaging continues to function when the storage server is unreachable, see Local survivability. You can also use Message Mirror to achieve a complete disaster recovery. For more information, see Message Mirror.