In moving from a single-tiered architecture to a multiple-tiered architecture, Directory Server should be the first component that you “split out” onto its own machine. At a certain point of load, Directory Server and Messaging Server on the same host have inherent performance impacts. This is due to the way Messaging Server is architected to work with Directory Server. Separating Directory Server onto its own machine is the first step to improve performance for a deployment.
See Chapter 5, Developing a Communications Suite Logical Architecture for more information on tiered architectures.
You can install Directory Server in such a way to have a clear separation between the directory user management and the software application configuration. Should you want to remove the software application configuration piece, this separation provides a cleaner way of removing that information from Directory Server.