Arsenal didnt need to go for it as they had the 1-0 lead from the first leg, Villarreal did.  Arsenal set out with 1 up front and a 5 man midfield.  I thought their defence was far from formiddable, was just that Arsenal never really got into gear, nor did they need to attack - as you say they were not getting careless and throwing players forward leaving ourselves vunerable for the counter attack.
was far from a great performance from Arsenal but they did the job.  thats 10 games in a row, in the Champions League, without conceding a goal.