In the studio I worked at, back in '04 we bought a G5 Xserve. It was used as a file server and email server. It worked pretty well... we could share using AFP, meaning no worries with special characters in the file names, and it gave us instant internal email, simply polling our web server for incoming mail every 15 minutes. We should have bought a G5 tower, in hindsight, because htat thing was huge -- deeper than most racks, and noisy as hell. It was like a plane revving up before takeoff, all the time.
If the Mini server had been available at the time, we'd have taken it. Strikes me that Apple has realised that that's where most of its server sales are -- to SMB's running Mac networks with some PCs attached, mainly for small workgroup activities: test web server, internal email, files, iCal, etc.
As for device speed, for anything other than a render farm, FW800 is fine. The average Gigabit Ethernet transfers files at about 30MB/s, which is USB2 speed, so USB and FW should be okay for that.
What's next? I expect they'll take their Xserve resources and use them to enhance the Mini Server as a separate product. With what they've fitted in the current model, a new one twice the height would have room for more RAM, a higher TDP rating CPU, maybe removable drive sleds, and a second PSU. Throw in some eSATA ports and you could have a Mighty Mac Server.
With the big push the Mini Server gave OS X Server, I don't think they'll be giving up on the software soon.
Oh and that data centre running Linux? Right. Because they don;t already own a server OS called Darwin.