The reason I ask is because I have never done that due to issues with the PDC failing and the BDC (backup domain controller) taking over, causing the complete loss of other system functions as noted above.
I completely built a domain with the PDC, two BDCs and two name servers. No other data or applications were allowed on those servers so they could do the job they were built for.
The decision of whether to consolidate services and applications onto a single server is entirely dependent on the resource load placed on those servers by your user base. I have had situations similar to the one you described, in which the PDC was the PDC and that was its only reason for existing. I've also had entirely satisfactory results running file sharing and network applications on domain controllers with sufficient hardware resources (RAM, processor speed, etc.) to handle the additional load. You are correct, however, in noting that a PDC-to-BDC failover will not do anything to fail over any applications that may be residing on the PDC. To accomplish this, you will need to implement some form of hardware or software clustering solution, either the Microsoft Cluster Service or a third-party solution.
This was first published in May 2003