That's exactly it. You can do it yourself for free if you'd like, but most admins would like not to. Granted you could always do Gentoo, but I think most admins would still go the way of the Red Fedora if it means round the clock support and updates served right to your doorstep.
You could serve everything up on Windows Server 2003, and then go with XP for all your workstations, but you'd first have to buy the server OS, then pay a license for all the client OSes, then pay more licenses for Office and whatever other proprietary software you use. An alternative is to buy RHEL 3, and use Fedora on all the workstations. The only license you need to pay for is the services provided by Red Hat. All in all, a pretty good deal!