Make it possbible to use ArcGIS server map services as layers in a map service. This would make it easier to maintain symbology when using the same symbology for layers used in several map services by creating a set of base map services. It would also improve speed when using several ArcGIS server layers in one client.
The benifit of doing it this way instead of having the client to call all the base map layers is that the client only has to perform one request round trip and only one image has to be transferred over the network. Most likely, the server is performing these operations much faster internally.