Back in the FMS 11 and below days, you were able to install Filemaker Server on a Mac OS X Server install.
Since FMS 12, Mac OS X Server is not supported anymore.
As an Apple subsidiary it's utterly ridiculous in itself, and that shouldn't be tolerated.
Bust mostly, Mac OS X server is useful, and companies can't always afford to have yet another machine to buy and administer just to get the OS X Server features. They'd rather put the money on a beefier Mac hardware, than having to put the money in another mac.
I don't buy the "FMS should be the only process running" stance as, unfortunately FMS, at least for non web direct usage, is never able to use all the ressource of the Machine. So there's plenty of ressource for the not so resource intensive Mac OS X Server processes.