I don't regularly shop in high-end, branded stores. Can't afford it. Thus I can't really speak with any authority about whether or not those stores carry higher-quality clothing than, say, mid-level department stores. I also don't frequent art galleries for the same reason. The art on my walls is either original art by the under-10 set (usually on paper; occasionally on the wall itself) or cheap (and I mean CHEAP) copies of "real" masterpieces.
Clothes are an interesting intersection, I think, of art and utility and commercialism - like furniture. I have attended art museum exhibits on fashion and have marveled at the creativity and skill that go into couture clothing. I am fortunate to have learned to sew from my mother, and I have experienced the creative process that goes into designing and making a dress.
I think it's hard to argue that couture is not worth its cost - those clothes that are designed and fitted and made to be one-of-a-kind. The line starts to blur, though, when you get into ready-to-wear. Once clothing starts to be mass-produced, the focus is less on the craft and more on the product. The clothes that come from high-end brands may be made from better materials and possibly by higher-skilled laborers, but they aren't originals.
At this point, then, I think it's up to the consumer to make a good choice. If you walk into Gucci (or wherever) and find a shirt that has a design that you love (and will wear for a long time), that has a flattering fit, and that has high-quality workmanship, then you decide what you are willing to pay for it, and either buy the shirt or leave it on the rack. Then, it seems to me, you've made an informed purchase. If you walk into Gucci, however, and think, "Well, it's Gucci, so it's gotta be good," and plunk down whatever they are asking for it, then I think you have fallen victim to the allure of the brand name.
I like that (and sorry to switch sides of the Atlantic, but I only know American stores) Kohl's, Target, and even Walmart(!) have clothing lines that are designed by some well-known designers (Vera Wang, Isaac Mizrahi, etc.). The designs are trendy and interesting, but the workmanship is, well, not good, but then again, they are cheap. Nobody is going to mistake my Vera Wang for Kohl's dress from an honest-to-goodness Vera Wang. They last about one season and then they fall apart, which is fine, because by then they are out of style. If I am looking for something that is less trendy and will last longer, I go to a higher-end store and pay more for it. If it doesn't fit right, I alter it.
If I want something timeless that is going to fit me perfectly and last forever, I make it myself. :)