wsm-pm application in enterprise deployment?

We're about to create some SOA and WebCenter environments that have clustering, and are following the enterprise deployment guides. We are puzzled by the separation of the wsm-pm application into it's own managed server. While clustering the application appears sensible, why do the enterprise deployment guides place it in its own managed server?

Also, when installing WebCenter, the wsm-pm application is targeted to both the Portlet and Spaces managed servers (but not the Services one), even if they are both on the same physical server. Both of these appear to work properly (ie the Validate Policy Manager link of both returns the correct list of policies). Can someone explain how the wsm-pm application works when it has multiple deployments such as this? Perhaps this will help us understand why the application is separated in an enterprise deployment.