Because noise can ruin a photo. It can add “texture” to where there should be none or little and can reduce fine detail. Some people notice this in the final result more than others. I am cursed with noticing it more than some. Noise also interferes with the colours in the image. As good as most modern NR programs are, they only guess at what the purple blobs would have been and subtle colour gradations can suffer. Go to some of the camera forums and see how many people complain about seeing noisy blue skies. Personally I don’t think a pic should be completely plastic but I’m not a fan of noise and don’t miss my iso400 film days. My situation is also a little different, having to shoot weddings in good light and bad ( in level and quality which is tough on a sensor) so I have to fight against noise more than many people do. I’m looking forward to the next version of the 5D series that will hopefully give me better results at iso 3200 and who knows, maybe even 6400.