If you want that particular breed, go for it. I'm an animal lover at heart, and I'd love to see every single shelter animal adopted. But that's not going to happen, and yeah you'd probably get a shelter animal that's just as great, but if it's not what YOU want then you shouldn't adopt one from a shelter. I have both purebred dogs and a shelter mutt dog, and we have 6 adopted cats.
Purebred dogs (and cats I'd assume, but I don't have experience with them)let you know ahead of time the general temperament of the breed. My family adopted a great dane when I was staying with them, a friend from my mother's work found a stray one and asked around if anyone was looking for a dog. My family ended up adopting it, and I fell in love with the breed. So when I was getting ready to move into my own house I saw an ad in the paper for a cheap dane, the breeder was switching from danes to something smaller and wanted to sell the last puppy. So I got my dane super cheap and she's perfect. My other purebred dog is a german shepard, the woman was moving out of her home so she wanted to find new homes for the puppies super quick. He's very typical of a german shepard, easy to train and kind of high strung. We love our mutt just as much as the other two, and ironically he has a lot more health problems than the other two. He has skin allergies, leg problems (including hip dysplasia), and he is probably the worst out of the three to train. But that's the luck of the draw, you never know what you're going to get, purebred or not.
Bottom line is you should get whatever type of cat you want. Adopting is great, but it may not be for you. If you know what you want, go for it. It doesn't make you a horrible person to pick the pet you want instead of adopting.
