Here is an answer and an opinion:
They have a server version because inadequate people apparently need to set-up servers too. Windows doesn't add anything to IT environment for experienced people with knowledge.
You can argue about examples of big companies using windows, but that doesn't change the facts about what someone with knowledge can do. For those big companies it's money that matters, and if they started out small, with windows,
they will continue to use windows because of the vendor lock-in. They can't go anywhere else, because they'd have to get rid of all the information stored up till now. There are ways to transfer/move/export, but there is no useful, fast, reliable way.
Windows isn't used for the technology, because there isn't anything in windows that you can't get anywhere else. It's always about the money, and quantity makes a product cheap. Windows isn't real quality, it's only quality to those who don't require much from their systems, because they are okay with the trouble, the resource management and the reboots, downtime and constant fear of something going wrong. (Those things are more like the problems as seen by the desktop users in the companies using windows server with windows clients - the system administrators have a different take on the whole thing, but hey, they just do what the big boss told them (Microsoft) and aren't responsible if it's not working)
Stuff like Active Directory (LDAP with extra's) and Filing services (Samba really isn't that efficient or fast) aren't that hard to configure on a different system. Exchange is a good concept, but it's way better implemented with DAV-based protocols, and maybe more imap-alike mailing systems. Microsoft's SMTP server is a bust, so let's not talk about that, we all know there are better things out there, but with Exchange you don't have a real choice. The deployment services aren't that special, and the update services are stupid as they are implemented right now. Most of the things done between Windows Clients and Windows Server like policies, remote application installations etcetera, are very easy to do on different systems. Most of the GUI-stuff done by the sysadmins on Windows Server systems are quite slow/lame and can de done way faster on a CLI shell, which Microsoft tries to implement with the PowerShell, which is too complex for beginners, and too limited for sysadmins who want to do everything from the CLI. The Windows Server version without a GUI sucks at best, because most of Windows's server software requires a GUI. Then there is no clean way for remote access, it's always KVM-like emulation, or some unsecured, slow, unsupported Telnet session.
It's something where specifically Linux rocks at, distributions like CentOS, Fedora, and Debian have everything
without requiring to get software from a third-party. You just deploy your server system via PXE, have it pull a designated configuration from your configuration server, configure itself and once it's up, add itself to your favorite monitoring suite. It's something that's done fast, clean, and easy. You can mass-execute things from a cron server via SSH, so it's secure and centralized, but decentralized and fast at the same time. Now with windows you have the automated installation, and there is windows via PXE, but it's all a PITA, even with the recent tools won't work consistently.
D'oh, I lost what I was going to write about, I had it all in my head but in the middle of it I had a small moment a la genius, which I wanted to write up elsewhere and quickly implement to test it. Now, when I rember what I was going to write, I will edit this post
Anyway, how is it you guys see this server item?
Oh, a quick addition:
About the software-requires-windows riddle, you can run most .NET stuff on Linux, you can operate most Windows applications in combination with Unix and Linux servers, and, there is the other part where employees 'need windows': fist, the argument about windows at home was that you used it at work, so you bought whatever you used at work for home usage. Now it's the other way around? That's stupid! On the other hand, it's even more stupid to not have interchangeable IT infrastructure. Binding yourself to one system or one vendor is the worst thing possible, you need to have at least a plan B and plan C, with everything in it.. If you don't, you will run in to problems eventually and die a slow and painful death. (Like with Adaptec RAID arrays, if your controller dies, you are in TROUBLE - software raid isn't the solution, but RAID arrays with open recovery solutions is, that said, having a system that decides for you if your hardware still works or not, THAT is stupid.)