It's not about being a plus defender. We haven't had a plus defender at shortstop since... on God I don't remember. Scutaro sure as heck wasn't one. Lowrie wasn't one. Nomar wasn't one. Renteria was a joke. I barely remember Valentin, but he was more of an offensive guy too IIRC. Cabrera and Gonzo were pretty good, but "plus" stretches it -- Cabrera was barely here for any time, and Gonzo is a journeyman with good hands who only looked as good to us as he did because of the guy he replaced, and the guy who replaced him, both SUCKING.
I'll take actual adequacy from the shortstop position. It's a big step up from anything we've had in a very, very long time.
I'll tell you this though -- if Iglesias ever makes the big leagues as our starting shortstop, we are going to be able to tell the difference -- just like we could when we got to watch Coco that one year when he really lived up to his ability.