I dunno. We know from "The Menagerie" that Spock can be intensely loyal to the point of breaking the law, but would he really be willing to place loyalty to a childhood mentor over loyalty to Kirk, especially when that mentor was a criminal and terrorist? I think I have to go with Shatner here -- the only bond strong enough to justify Spock surrendering to Sybok once they got aboard the ship was the bond of family.