I never understood why some people were against Spock having a half brother.
When DC Fontana was working for TOS, she became the custodian for all things Vulcan, and had put out a memo suggesting to the writers that, in order for Spock to retain his uniqueness as a character, they should refrain from doing stories where all manner of siblings appear as guest stars.
She and Roddenberry reminded Shatner of this when the script of ST V was being firmed up, but he chose to ignore their input. DC also managed to put a cheeky line into her then-upcoming novel, "Vulcan's Glory", where Amanda states that Spock is "the only son of Sarek". (A few months later and the line would probably have been removed.)