Indeed, having Sybok be Spock's childhood friend could well have been momentous for our poor half-blood. After all, Spock might well have been the first Vulcan in post-Surakian history to have a childhood friend.
I wasn't much bothered by Sybok's ability to take over the ship even when our heroes had multiple options to defeat him. After all, that was specifically Sybok's knack: to make people forget that they have options, agendas and priorities, and have them proceed in a manner suiting Sybok's plans instead, or then not proceed at all. Of course, Sybok wanted to call that "freedom", but it came out as the exact opposite.
It's probably a Mandrake the Magician thing: you can't defeat Sybok if you come within eyesight of him, but you can start planning jailbreaks and the like if you stay far enough away from him. Vulcan telepathy is typically described as range-limited anyway.
I wonder if Sybok's "special" ability is in fact standard fare but considered shameful by the Vulcans. It could be the very same thing Spock uses on the chick in "Omega Glory" - a telepathic flirtation technique that a Surakian Vulcan wouldn't be caught dead using in public.
No, I don't consider Sybok a silly idea in any sense, and I very much like all the aspects of the practical execution, from casting and makeup to mannerisms and dialogue and plot. This character could really carry the movie if the massive pacing problems and the shortcomings of visual appeal could be overcome.
Timo Saloniemi