My main arguments for Arnold in the test team have been that he is a strong lineout operator, mobile around the park, brilliant at disrupting mauls, and, crucially, forms a good partnership with Coleman.
Rodda has him beat for sheer work in making tackles, and is probably a better scrummaging lock. Carter can match him in this area, but, unlike Rodda, doesn't form a good partnership with Coleman.
Arnold leaving next year does force our hand in a way; we can't bench our hopes in the guy long-term, and the gains and losses in picking him instead of Rodda or Philip are marginal. I think he's good enough, but not so much so that he is demaning starts.