Unfortunately, the human condition is predisposed to creating these environments. People tend to take the path of least resistance. Why trouble to "see the world" when you can marry "the girl next door". It's easier to run the same operating system on your firewalls as you do on your workstations. It's easier to train your users to run the same word processor, whether it's unfriendly to every other WP or not.
@stake, whose origins were not exactly related to a business plan, "sold out" (IMO <-- for those litigous natures) long ago. Mr. Geer was fired because his opinions conflicted with someone in charge. (Hint: Companies don't have opinions. People do. He was fired because he angered someone with the power to do so.) (I hope he sues because he was expressing concerns about a security issue while being employed by a company which specializes in security.)
And before you put me down as being anti-MS, let me state that I'm not. Rather, list me as a member of the "the best tool for the job" crowd. If you're running MS on your desktops, you'd better be running some version of commercial Unix on your firewalls and some other version of *nix on your NOC equipment. The larger your customer base is, the more important this is. Diversive network equipment, while requiring a wider talent-base (read that as $$), is more resistant to inbreeding and failure in the long run.
[Oh and, yes, you can put me down as implying that point-and-click administrators have narrow family trees. Eventually it leads to "Hey, what's this button do?" and "Hey, watch this!" (Which leads to family-hour comedy shows. But that's another story.)]
Note: Philip Greenspun has a post on the same topic. I'm especially entertained that "ass ugly" is a logarithmic (Gaussian) scale and that the majority of system cases are a .05 deviation. [I wonder if he ever saw the attempt to sell cube-balanced-on-a-corner systems to self-styled power geeks [okay, posers!] (circa 1998).]