For me, the design of a phone DOES affect its functionality. If the camera is placed in a weird area, it makes it difficult to take pictures. If the power button is on the side, in the middle, it makes it difficult to turn on/off the phone. If the headphone jack is on the side? Same thing. There's a ton about design that isn't just "herp derp, its pretty!!".

Same thing about the plastics used. I don't want a phone that attracts dirt and fingerprints, because then the screen will be harder to look at, or the back may feel odd to the touch. Maybe a metal phone is a bad idea, since the phone may heat up. Glass? Will shatter when broken. There's so much to consider in a phone's design that it's a huge factor in my purchase.