When in fire-at-will mode, will turreted screening shell frags automatically engage incoming torpedoes and missiles? If so, this would be a good defense for traders and gank-evaders.
This question is asked from time to time but unfortunately the answer is "no".
Not worth it.
Either run away or equip to kill.
You could equip a point defense, but it's situational while shield boosters are universal.
Fire at will is for targetting ships only, while the screening is for munitions. They don't mix sadly so like others have said fit something else.
The other issue is you'd run out of ammo almost instantly (there is a reason why the PD has 10000 ammo).