That bothered me a bit too. I wonder if it had to do with just wanting to get Spock cured and doing anything possible to make it happen. They knew they had killed the creature, so putting Spock in there next might have been excitement taking over?
The Doomsday Machine
Now this is a Star Trek action show done really well. Loved how manacing the Doomsday Machine was, and how at the end they even brought in some subtle commentary on nuclear weapons and how bad they were. However, this episode is not my favorite episode of the series (Maybe a step below Mirror, Mirror) because of one reason, how easy it was for Decker to take command. Ok, so regulation states that you had to provide a written medical examination to prove the guy is fit for command, yet he had just gone through a tramatic experience in losing his crew, he shows he's not all "There", and they still need proof? Shouldn't common sense and "logic" take over for the safety of ship and crew? I kind of wish Spock put up more of a fight and maybe even stood behind McCoy here.