Tuvix was worse. Transporter accident created a new life from two individuals. They should've accepted this change (Federation Principles after all) and not pursued the course of separating them in to two individuals again.
With Sim it wasn't just his life that was at stake at being lost it was Trip's. It's reminiscent of the movie "The Island" with Ewan McGregor and Scarlett Johansson. Clones were created by the wealthy and their bodies, and organs were used for a multitude of purposes should their original need them. However Sim only had a short life expectancy to begin with. It's not like "The Island" where the clones were capable of living full lives. Sim had weeks if not months to live a full life before he would expire. I can understand the logic behind it. It's similar to what we have today with organ cloning and transplants. Sim just happened to have a mind and could think and make decisions. If he hadn't he would be no different than a cloned liver that would be transplanted into an alcoholic who has used and abused their original liver.
Not squeaky clean but given Archer's situation I think he was justified to allow Pholox the go ahead with the cloning. Had Sim not volunteered or ran away like he planned then Trip would've died and Sim would've have also.