If I recall, Archer did say that if they were in peacetime he would consider the option to try and save Sim but he didn't have that choice because the entire planet was at stake and he couldn't complete the mission without Trip, it was a case of the needs of the many...
BTW, simmilitude was a much better episode, than Tuvix. I actually shed a tear haha
I shed more than one
I don't remember the Tuvix episode to comment on it.
However the Similtude creation was done for the greater good. Also his natural lifespan was going to be 15 days. Phlox genuinely thought they could operate without it resulting in Sim's death...