I think the point isn’t so much immutability for the sake of immutability but rather immutability for the sake of keeping power out of a centralized entity. With the scheme proposed in this article, you could do things like have contract changes only be authorized by, say a 2/3 vote of token holders, or any other complex logic you want to implement in a smart contract. Of course if there is some contract that has a single individual with the power to make arbitrary modifications, it likely won’t be successful, but with decentralized ownership I think it could work.