What? Sybok was a full Vulcan. We were told in the film that he was the son of Sarek and a "Vulcan princess." And since we know that Vulcans are betrothed as children, it's reasonable that Sarek had a Vulcan wife before he married Amanda.
I didn't mean Sybok wasn't full Vulcan. I meant he was Spock's Half-Brother.
I thought the idea of Spock suddenly having another family member out of blue was bad writing.
And the whole "emotional" thing didn't work for me either. In fact there was nothing Vulcan about Sybok at all. Not that I am saying every Vulcan has to be like Spock but there was nothing that made Sybok seem anything but a zealot of a human, not Vulcan.
From reading Leonard Nimoy's autobiography I got the notion that he also didn't like the idea as when he talked about his problems with the Final Frontier script he mentioned it with a list of problems (religious zealot who just happens to be Spock's 1/2 brother).
And Sybok's rogue philosophy helps explain why Sarek disowned Spock for going into Starfleet, why they went 18 years without speaking as father and son. Maybe Sarek was afraid that Spock was following the same path that Sybok had and would bring similar shame to the family.
It might explain it but it doesn't mean this was the "best explanation" or the only explanation possible.