I agree that many cyclists feel a degree of devotion or attachment to their accessories that can't be adequately explained if we look only at the justifying arguments they give. So we need to cite other psychological factors. Group identification may, as you say, be a motive for many people. Deference to other authority figures, such as salespeople in bike shops, is important too. I remember, as I was getting into cycling, the subtle but relentless message I got from the folks at my LBS: "There's nothing wrong with street clothes/sneakers/these stock flat pedals; but when you're ready to take your riding to another level, you'll want padded shorts/dedicated cycling shoes/clipless pedals." Wow, was I ready to work hard on my new bike in order to earn the right to these enhancements! And for a long time after I started dropping the cash for all that new stuff, I convinced myself that they were unambiguous improvements, and that there could be no good reason to "go back."

We are very good at constructing ex post facto rationalizations of our decisions. I remember riding my road bike down to the shop for repairs. I was wearing street clothes, and I was saying to myself, "Does anyone actually like wearing street clothes on a bike? If only those poor saps saw the light." It was only when I started carting my daughter around that I considered ditching the racing gear. At this point I have a hard time re-imagining all the fuss. Perhaps that's why I've found this thread so interesting. We've seen a lot of salesmanship here, but rather less of folks like Tom (noglider), who's happy to say nothing more than that he loves his SPDs. No implication that his reasons ought to be your reasons; he just loves 'em.