The complex systems all predict his offense to be below average and his defense to be very good. They seem to be a bit skewed by the fact that Arcia was a much better defender early in his career.
But Arcia's defense has eroded big time. He's nowhere near the defender he was when he first came up. He's been below average the last couple of years. If he's below average both offensively and defensively, it's unlikely he'll reach his WAR predictions.
Maybe he turns it around. Maybe he suddenly becomes a better hitter or fielder. But there's not much reason to believe he's a guaranteed 1 WAR.
But also, why would we be happy with 1 WAR? That would rank as the worst starting SS in the majors last year.
Because his contract would have surplus value and when you a loaded and rapidly increasing expensive roster having someone like Arcia to hold the fort for 2-3 years while you hope to develop another young player is very valuable.