First, we live in a very imperfect world. Heterogeneous networks are messy, messy things. Sure there are protocols for Active Directory, Open Directory, LDAP, DHCP, etc., but vendors do one of two things: fail to implement the spec properly, or add a bunch of proprietary bells and whistles that aren't part of the spec, are technically add-ons, but still seem to mysteriously cause hardware or software to fail when they aren't present. Throw a couple of DNS forwarding issues, some CISCO equipment and maybe a Radius server into the mix, and things get ugly fast. What's the admins final line of defense against complete network chaos? Hardware addressing. Figure out what hardware is sitting at which MAC address, and build policies based on that. It's not ideal, but it's the the way the real world works. If you can't predict the OS type from the MAC, your job becomes 10 times harder in a flash.
Second, nobody actually wants to reboot. It's time consuming, stressful on the hardware, and just generally not too much fun. It also means getting users in the habit of interacting with the firmware, which is something sane sysadmins want to avoid at all costs. What admins, and others, want is real virtualization. Not dual booting. Not emulation and compatibility layers. Real virtualization. When Apple delivers that, there will be partying in the streets.
Third, there's no support and it doesn't look like there's ever going to be. Unlike the rest of us peons, large education and enterprise clients spend a lot of money on premium AppleCare services. They have reps who know them by name, and part of what makes Macs appealing is that you call one number and get integrated hardware and OS support. If Apple won't support Windows, dual booting will mean buying a second support contract for the same machine. hat more than negates the cost benefit of a single machine solution. Beige boxes are cheap and procurement already has contracts with HP and Dell. There is, of course, a potential for third parties here to step up and become Apple Authorized Resellers offering pre-configured machines with support, but that's a niche market. Most organizations that buy Macs want to deal directly with Apple.
And then for education tech support, there's the added fun of personal machines that people use to connect to the network....
Individual admins, of course, are thrilled. Being able do dual boot, say, a MacBook Pro means only needing one machine to administer everything. But supporting it for users? That's a different story.