|Don't ask to ask, just ask|
They do it for some of the same reasons that people cheer on sport teams.
Let us not forget the origins of the word "fan" (Fanatic for those wondering).
Partly to belong to something.
Partly to justify one's own decisions.
For some its more about being right.
For others its about being first.
However, I think one of the main reason for the Zealotry is simply because its easy. If you have never used, seen or even heard about a particular new language, methodology or gadget that a Zealot is touting, then how well prepared are you to challenge it's validity? Most Zealots, that I have encountered, are of the type that gloss over the details, greps for buzz-words, then proselytize from the nearest mountain top, secure in their knowledge that they may not know everything, but they know more than you currently do.
One way to prove yourself right is to prove everyone else wrong, this is generally when you get the "You just don't understand..." comments, a weak attempt to cut to the case and just declare you as WRONG!... save all the time arguing. human nature can be so silly some times.
"I do not feel obliged to believe that the same God who endowed us with sense, reason and intellect has intended us to forego their use." -Galileo