It would be great if KO had the physical intangibles of Smart, Crowder, AB, Amir, etc, but he doesn't. He is a great shooter and positional player, but if Horford and Millsap are going to take a shot at his shoulder every time down the floor, then Kelly will be even more tentative. His solid shooting and passing would take a drastic turn for the worse and his rebounding and boxing out would nearly disappear; he would look ridiculous out there.
It's easy to look at pasty, long-haired Kelly and think, 'wuss', and maybe he isn't as rugged as some others, but if he can't use his skills, then there is really no reason for him to be out there.