If you want an interesting and informative view on what you're talking about, I suggest reading the book below. Cialdini in his book talks about seven phenomena which play into a person's decision making and the effect other people have upon influencing another's decision making, and committment to a previous decsion is one of those factors.
The annoyance you're talking about actually goes way beyond computers and speaks to a fundamental psychological phenomenon. When someone makes a decision, they find reasons to justify the switch they made. Almost any recent convert in anything from computers to religions is going to be really excited about their new situation and critical about where they were before.
What you're talking about with recent Macintosh converts you'll see in different forms in everything from nonsmokers to born-again Christians. Regardless of the change being made, there was some decision behind it that was committed to and psychologically we find ways to reinforce that committment to ourselves.
To some degree it's human nature and it can't be helped, but I do agree with you that it would be nice if some of the more excessively zealous folks would tone it down a bit, and it would *REALLY* be wonderful if some wouldn't find it so necessary to bash on other people's choices to justify to themselves the decision they made, and sometime you wish you could grab them by the scruff of the neck, throw them in a bathroom stall and toss a ruler in to have them settle it once and for all (for some reason it's been my experience that this is almost exclusively a guy thing).
There are things to like about all the platforms, and each has something it excels at over the others. For better or worse, Apple has made evangelism more a core part of their business strategy so you see a bit more of it with Mac's than with other OSes, but each group has its boosters (and we all know who they are). However, in any event it has less to do with the machine and more to do with the owner.