I totally disagree, Spock would have been logical in his decision and it is not logical for the most valuable member of the Galileo crew to stay behind. It would not have been Spock, McCoy or Scott staying while some ensigns took off back to the Enterprise; it's not logical. Now Kirk, he would certainly stay!
Side note, in one of the early drafts, they were actually drawing straws to determined who stayed, which was mentioned in passing as a possibility in the filmed version. Also, it was originally Kirk who piloted the shuttlecraft with Spock remaining on the Enterprise.