Windows 2000's Native Mode blurs the distinction between a Backup Domain Controller and a Primary Domain Controller....
In the past, PDCs managed replication of the security database using a "master-slave" relationship. Native Mode moves to a multi-master model meant to improve domain-replication performance greatly.
If your network has only a single domain controller, then by all means create a mirrored drive of that controller's boot drive and store that mirror for future use. In case of disaster, your spare is a simple insertion away (assuming you have a hot-swappable system), and your system is only a reboot away from being operational. Every so often consider reestablishing the mirror to update changes. With that mirror in place you now have time to recreate the most up-to-date disk from backup media.
Another instance that makes having a replicated domain server valuable is the establishment of a remote office. Replication over slow links is often impractical, but it is certainly practical to send the replicated drive of a domain controller to the new office. But be careful. That drive contains your company's security database, and while it is not easy to hack into the information, it is not impossible.
Barrie Sosinsky (email@example.com)is president of consulting company Sosinsky and Associates (Medfield MA). He has written extensively on a variety of computer topics. His company specializes in custom software (database and Web related), training and technical documentation.