Since Apple permits OSX Server Edition to run in a Virtual Environment, there isn't much impact for large users of Xserve 1U's.
You can buy much faster, cheaper, industry standard 1U's with more features than an Xserve for half the cost.
Put a rack (or 1000 racks) of those cheap 1U's in your data center. Run VMWare, or your favorite hypervisor flavor, and make as many OSX Server virtual machines as you want.
There is no special port, or "reality field compensator" gadget in the Xserve hardware. So, regular Intel based 1U rack mountable servers are just as good.
Now, the cool thing is, you can take your rack of 10 servers, and decide that today you'll have 5 large Linux servers, and 20 medium OSX servers. Yeah, see what I did there? In virtualization, your number of virtual servers depends on utilization, not physical hardware. Later that week, someone wants to test out a new customer service Wiki, so you click a couple buttons and magically, 1 small OSX server is added to your virtual pool for testing the new Wiki. No need to rack and spin up a new server.
Virtualization is powerful, cost efficient, and means expensive special built rack mountable servers are obsolete. Fill your data center with the cheapest, fastest, redundant commodity hardware. Then add as many servers as you need running what ever operating system supports your apps.