Sure. Lemme get out my copy of FreeBSD Server Edition (TM), rather than Desktop Edition (TM)...err, wait. There isn't any such thing.
I can sort of understand segmenting your clientele based on the number of concurrent connections, or the number of processors, or some such. I'm not so sure segmenting based on security is such a hot idea. Unless you're suggesting something other than that OS X server is more secure than the desktop version.
It's as suitable as any other OS in its market. They ALL have had privilege elevation exploits.
Which one do we know for a fact has unpublished, unpatched exploits TODAY?
Not for FreeBSD, but there is for OS X. Since we don't know the exact exploit used, we don't know if it works on Server too. This isn't as big as the difference between NT4 (server) and Windows 98 (client), but it's bigger than the difference between NT 3.51 Server and Workstation (the only difference there being registry entries that restricted Workstation, Microsoft made it a bit harder for NT 4 but it's still doable).
Face it, all OSs are a race against hackers. At any one time, any OS will have known and unknown vulnerabilities, patched and unpatched. What matters is the reaction time to fix once notified and the number/severity of the vulnerabilities for the roles you're using. Anything else is just playing games.