Although, there is one little tid bit of information that one must keep in mind. It was only during the battle between Ichigo and Ulquiorra that we discovered that the Arrancars have the capability to ascend to another level beyond their initial release. Ulquiorra's second-stage release, which he dubbed the "Ressurecion Segunda Espada", showed just how much power an Arrancar is truly capable of. Ulquiorra in this evolved form was powerful enough. It doesn't take much to realize what would happen if all Arrancars, or even just the Espada themselves, were capable of this release. I imagine that Kubo had to keep them to their first-stage release to give the Soul Society Captains a fighting chance.