Sometimes, I am asked why there is no single version of Windows that contains everything. Instead, as you move up the ladder, say, from Windows XP Professional to Windows Server 2003, you gain server features and lose workstation features. Why lose features when you add others?
Because it turns out no actual customer wants to keep the workstation features on their servers. Only developers want to have this "all-encompassing" version of Windows, and making it available to them would result in developers testing their programs on a version of Windows no actual customer owns.
I think one of my colleagues who works in security support explained it best:
When customers ask why their server has Internet Explorer, NetMeeting, Media Player, Games, Instant Messenger, etc., installed by default, it's hard for the support folks to come up with a good answer. Many customers view each additional installed component as additional risk, and want their servers to have the least possible amount of stuff installed.
If you're the CIO of a bank, the thought that your servers are capable of playing Quake must give you the heebie-jeebies.
[Raymond is currently away; this message was pre-recorded.]