Well, there's two questions here:
1) Are vegetables necessary for good health? Like I said in the other thread, that depends. For some people, complete elimination of plants is what's necessary for their good health. Everyone has different genetics, different degrees of metabolic damage, different environmental factors. And everyone has their own hierarchy of values by which they decide whether various choices are "worth it", for whatever reason.
2) Does Mark Sisson advocate vegetables/plants solely to make his plan less scary for the average person? That's a complex question that only Mark can answer, but based on his writings, I would say that he's perfectly aware of all the arguments for and against plants in the human diet, and on the whole believes that thoughtful addition of such is more beneficial than harmful, both physically and psychologically.