I commend this article by Professor J.E. Lendon on the Roman siege of Jerusalem. Drawing heavily on Josephus, who was an eye witness, he make some very good points about the use of Auxillia for combat because of their Virtue of Courage, and the Legionaries because of their discipline and engineering skills. He also speak of the constant training of all Roman soldiers. This is a lengthy article, but one I endorse to all those who seek to understand the legions of the 1st Century.
IN HOC SIGNO VINCES - TIME DEUM ET OPERARE IUSTITIAM