The big missing link in Valeris's arc is motive. While being half-Romulan doesn't inherently make her a traitor, it does offer reason as to why she might do it. And having her as the "new guy" just weakens the whole thing even more. But then I suppose it's better than if they had made her a red herring.
The motive is the same as Cartwright's. She logically saw it as a chance to bring the Klingons to their knees, thus eliminating a potential threat to the Federation at a stroke. She might have had a starkly different interpretation of logic to Spock - much more utilitarian - but that doesn't mean her actions weren't logical to her.
The scene in Spock's quarters basically reveals that - Valeris sees logic as the be-all and end-all, whereas Spock has the experience to understand that logic is only a part of wisdom. There's no reason to believe that all Vulcans are pacifist vegetarians like Spock is there?