But can/will Arias hit? I think that's still an unanswered question.
You're sorta answering a different question than I am. If Arias and Gimenez are both going to be pieces in your everyday starting lineup then yeah, you just play Arias at SS and Gimenez at 2B since Arias is a good defender. But I'm thinking of it from a team building standpoint, I'm not assuming that Arias (or anyone currently in the organization) is going to be the other middle infielder.
If you have a player signed long term who you like and who is excellent at multiple positions then I think that generally speaking the way to maximize your overall value is to use him to lock down the position that is much harder to fill (or at least remain open to the idea) and try to find a fit for the position that is easier to fill. It's easier to find a 2B (or cut and paste over the spot) than it is to find a reliable SS.