We have missed the old "good times" when Exchange had this own directory implementation: Exchange 5.5 "DS"-Directory Services, the first X.500 directory incarnation from MS.
-No needs of separate servers/services;
-Buil-in replication;
-Local resolution of Exchange Services queries (no network problems);
-Exchange Administrators with total control of Exchange roadmap;
-No needs to take care about globalwide 'schema' extensions modifications that can affect the entire organization.
And how about DNS? It is a completly independent entity, used
by AD and by anyone who needs it. Even MS-AD can run against non-MS DNSes.
Active Directory was born from Exchange 5.5 DS. And now the creature sometimes scares his creator...