Greens keepers know as well as anyone else that there is no rule against using something other than a putter on the green. The game is designed to score as low as you can, using the best club for the shot at hand. Yes, you should try to do as little damage (if any) to the green as possible. But as mentioned above, the design of the green contributes to club selection based on where the golfer's ball ends up on the green.