I find it quite annoying when people insist on treating their dog as if it were a child. I have no problem with putting clothes on a dog for special occasions as long as the dog doesn't become distressed by it but it's not natural behavior for a dog and it can have negative effects if done too often. A dogs coat alters depending on the temperature and if they are constantly stuffed into silly outfits then their fur doesn't have the chance to adjust. Dogs need to have the means to fulfill their natural instincts and silly clothing can restrict that ... besides I always think other dogs are laughing at them!
Having said that I have a whippet cross and he owns a coat for wearing outdoors when the weather is cold. The reason I bought it isn't so much that he shivers but that he looses a lot of weight when the weather turns bad. He only wears it in the middle of winter or when we are camping out but it's just for his health and not to look pretty or cute!