If you're going Last Word, you're going to have to put points in Arbiter anyway.
But talent-to-talent, Art of War's effectiveness is diminished because it empowers Exorcism, which really only hits decently hard if you're specced into Blazing Glory (+20% damage) which you won't. For an expensive 3 points, it's why it's one of the first talents to get axed. Likewise, Templar's Verdict hits as hard as it currently does due to Crusade (+30% damage), glyph (+15% damage) and Arbiter (+12% crit chance). In pvp it's all about stacking (I think the same can generally be said for pve too).