Hi Ben,

SerialSubscription and MultipleAssignmentSubscription are different things.  MultipleAssignment simply enables you to swap out the underlying subscription in a thread-safe manner.  Whereas, Serial unsubscribes from the previous underlying as you replace.

See:
http://msdn.microsoft.com/en-us/library/system.reactive.disposables.serialdisposable(v=vs.103).aspx
Vs:
http://msdn.microsoft.com/en-us/library/system.reactive.disposables.multipleassignmentdisposable(v=vs.103).aspx