I think that you will want to....
- ensure you have proper (and occasionally tested) backups of your Open Directory data, as OD does occasionally get corrupted, disks do fail, server security does get breached, and accidents do happen.
- establish an OD replica if/when your local operations become or are large enough to require that your authentication needs better uptime, or if you're starting to distribute your authentication data across multiple (remote) sites.
- consider an upgrade to newer versions of OS X Server, as OS X Server 10.6 has largely fallen off Apple support.
Is running one Open Directory server and no replicas particularly problematic? Not really. I know a number of sites that have services (DNS, OD, mail, whatever) that are effectively single points of failure. But that's as much a business decision as a technical one, and business decisions are almost always local decisions.