The best way to explain it would be to make a stereo system analogy. Whereas most people would not want to buy too many separate components, most users want to separate out the receiver (tuner / amplifier) from the other components like tape decks, CD changers and miscellaneous input sources like camcorders and VCR or TV feeds, and also from the speakers. Likewise they would prefer to buy a separate TV and VCR rahter than a combo which would prevent the user from being able to watch TV if the VCR section failed and needed repairs. The public would howl in protest if I bought say a Panasonic tape deck which was designed to work only with a Panasonic receiver or amplifier, and not the Harmon-Kardon or Kenwood that you might already have. If one component goes bad, you want to be able to replace it without necessarily having to use the same brand, and still have the rest of the components work.

The same goes for computer networks. It doesn't matter whether it's Unix, Windows, Macintosh, OS/2 or whatever -- computer programs have bugs in them. If the bugs become intolerable, you want to replace a particular program, whether it's the operating system on the server, or a mail program or other application without bringing down the whole network. Unfortunately Microsoft is a major offender in this area. A perfect example is Microsoft Exchange. One of its requirements is that it MUST run in an NT domain. This means that NT Server (or 2000 Server) MUST be installed on another machine OR you must make the Exchange server your domain controller (which makes your organization too dependent on one machine for too much). Samba domain controllers will not properly accept NT workstations and servers in a set of signals that the NT machines will acknowledge. So what if an organization likes Exchange but believes its NT domain controller crashes too often? You have to replace TWO components because of the failure of one. This is a major reason many large organizations continue to use high-end Unix platfroms for their server backbone, and only use applications (which include many Microsoft products) that are compliant with an open-standards architecture.

Don't get me wrong -- Microsoft makes many products that DO comply with non-Microsoft products. But they still make some that do not. Exchange and 'Internet Connection Sharing' are two. And a lot of their software is only "semi-open" meaning that it will run on only ONE other non-Microsoft operating system, namely the Apple MacOS.