Adequate is not enough for some. For them it is also additional value to stamp themselves as members of a certain tribe. Witness the SLR camera owners who must have Nikon or Canon emblazzoned on the neckstraps. There are endless examples of such brand loyalty.
You're not much of a photographer, are you?
Aside from the fact that the branded straps are included for free, as edavid already said, this is a VERY poor example of unjustified brand loyalty. In the film days, what made the biggest difference in image quality was the lens and the film stock. The body really just held those together. But today, it's the lens and the sensor data processing that makes the difference, and frankly, not every camera maker is good at the latter. Nikon and Canon a) make absolutely spectacular lenses, better than most, thanks to their experience in making high-tech lenses for semiconductor manufacturing, spy satellites, microscopes, etc. and b) have top-notch processing. Sony, for example, isn't as good at either, but fails especially at the latter. (As evidenced by mounting a good Canon or Nikon lens on a Sony and producing far inferior results.) This despite the fact that Sony is exceptionally good at making the sensors themselves, with Sony sensors used in a huge percentage of camera brands and smartphones. Leica is as expensive as it gets, but has absolutely wretched processing. (FWIW, many photographers think Apple has some of the very best in-camera processing, at least for full automatic modes.)
Additionally, once a photographer has decided on a brand, switching to another is very expensive, since it requires re-buying all the stuff you already own. It's far more pragmatic to stick with the brand you have. (And with the sole exception of Tokina, third-party lenses make no sense: They're cheaper, but not as good, and with FAR, FAR lower resale value than original brand, making the TCO worse in the end.)
So yeah, SLRs are, if anything, an example of where brand loyalty is very much justified and pragmatic, not the "brand loyalty" of the kind seen in clothing, for example.