I know exactly what you mean Alex. This sort of thing is all over the internet and it's irritating... but to experience it in "Real Life" like you describe must be even more annoying... that behaviour is really irrational; people don't normally wander up to your car window and say "You don't want one of these, you want a Ford", or approach you on the train just to say "Yeah I wouldn't use that phone... I only buy Samsungs". Photography may be a smaller niche but the idea of wandering up to someone just to criticise their equipment is really bizarre.
As for why you should care... well personally, I know I shouldn't. When this happens it doesn't make me question the value of my equipment or wish I had someone else's. But like I said, to me it seems so irrational... a little friendly competition between brand users is good, and comparing tests or reviews might be helpful to some... but to have someone go out of their way just to say "You have the wrong name on your camera", well yeah that's annoying. And like Alex says, often it isn't just the occasional person but lots of people a lot of the time.
As for why you should care... well personally, I know I shouldn't. When this happens it doesn't make me question the value of my equipment or wish I had someone else's. But like I said, to me it seems so irrational... a little friendly competition between brand users is good, and comparing tests or reviews might be helpful to some... but to have someone go out of their way just to say "You have the wrong name on your camera", well yeah that's annoying. And like Alex says, often it isn't just the occasional person but lots of people a lot of the time.