The original idea of the need for the Vulcan training in emotional control was that without it, the strength of their emotions would overcome them, and that they were naturally violent and passionate. But I was thinking that we have seen Vulcans who do not live according to the stoic philosophy of their culture- Sybok for one of course, and there's an ENT episode where we see a group of them(is that "fusion?" I can't remember the name).
Whatever criticisms of Sybok you want to make, it seems obvious that his issues were with his religious fanaticism and ego, not his temperament. He didn't become violent as a result of his rejection of Vulcan philosophy, rather the opposite. He was depicted as quasi-pacifist.
And then we have the Romulans, who are essentially Vulcans biologically and genetically speaking. And yet that civilization doesn't constantly erupt into civil wars and violence. They're not exactly models of egalitarian small-d democrats of course, but their societal model doesn't seem to have much to do with not following emotional suppression.
So why do Vulcans REALLY hold to the need for emotional control? It seems to be more of a philosophical preference along with the desire for logic rather than a "we must do it to keep from surrendering to violence" reason.