I think whether to buy multiples depends on a few factors. If you are hard to fit or have a hard time finding something that works (e.g., comfy shoes), buying multiples is helpful. I have also bought multiples simply because a hard-to-find color (for me, teal) was available, and I wanted that color in my closet. If nothing else, the multiples serve as placeholders until you can find something else that is different but does the job just as well.
I think the decision also depends on your style persona. If you are very set style-wise and know what you like, multiples work well. But if you are someone whose style persona changes a lot--you are a bit of a chameleon--multiples might be a downer over time because they will hinder the variety in your closet.
I go back and forth on the multiples issue, because I have a very changeable style persona, yet I am terribly hard to fit. I agree with Laura that I find two colors of the same item to be better than three--with three, I feel kind of "boxed in" and like I'm a robot. But I also started out this year with very few clothes, and if I hadn't bought multiples, I would have had a lot harder time filling my closet. I will also say that the multiples I bought are in very classic styles, so if it takes me a long time to wear them out, that's OK.
I guess I would say that I wish I didn't have to buy multiples, since I like a lot of variety. But practicality in my case dictates that I need to buy them some of the time.