I don't, generally because I like to try all kinds of different things. I'll never know how that RL shirt will fit or last if I only buy the shirts from BB that I'm comfortable with. I wouldn't look down on anyone who buys a bunch of the same thing, but I personally like to have a little more variety, in style, colour and design. And while I own several pairs of shoes from Alden and A&E, I wouldn't buy two of the same pair. I have one pair of Park Avenues for example, and have no desire to buy a second. Not that it's not a good shoe, it is, but I just don't need more than one.
I do this with clothes all the time, when you find something that fits and works well, why not take advantage of it. I have similiar Incotex/Mabitex pants in the same fit in multiple colors. Now that my shirts are set in sizing, I use 2 collar styles and just vary my patterns.