The only possible reason I can think of why someone would want to run an OSX server is if they were going to be remote-accessing it to run Xcode for iOS development. What else can you do on OSX that you can't do on Windows or Linux?
Beside iOS and OSX development, I don't see clear advantages of using a Mini as a server when you already have a good Linux setup and provisioning for other services.
At my company, we use them to run continuous integration for iOS and OSX software. We've got like 10 of them for this purpose and they work quite well. I think the oldest one even out-survived the oldest Linux server used for CI. We don't need all the fancy server features for them (ECC, redundant power, serial console, hardware RAID). If one dies, well, we shut it down and replace it with another. They are inexpensive, and just build servers anyway, not critical for the company.
They're not always easy to work with though, we had them in a DC, but our remote hands wouldn't touch them. Since we couldn't find any IPMI or KVM suited for them either, we decided to move them in a secured room in our office. We never needed to do anything of them anyway, but it's still handy to have them close by.
If anybody has a solution for remote KVM or IPMI on a Mini, that'd be great!