I've never really been keen to sticking to a single brand for clothes, because my style is so different. I don't really pay attention to brands when it comes to buying clothes, but I would not buy anything of bad quality. I think if a person belongs to a certain subculture, they would be more prone to sticking to a specific set of brands.
I don't call myself "loyal" when it comes to fashion brands. All I look first is if the product is inexpensive. Second is the look. Does it look chic or cool? Is it worth my taste? Next is comfort. If a piece of clothing looks chic and is comfy, but is very expensive, I won't even bother to give it a second look. Sometimes, when the price to pay is something I can put up with, I buy it, especially if it is also comfortable. Most of the time, though, I look for inexpensive clothes even if it's not a well-known brand.
I’m more loyal to a look than I am to any brand. Some shops and brands do serve my aesthetic better than others, but I’m willing to shop around because you never know what you might find in an unexpected shop or brand. If it’s decent quality, within my budget, and it coordinates overall with my style, I probably like it.