Your pios are probably where you last ordered them to go? You can also use control groups for your repairing units when you have vehicles in the field, I know I do. You just U-click your tank to where it isnt dangerous and order your pios to repair, takes 1 sec at most. Effectiveness of artillery imo has nothing to do with vehicle repairing, its not like allied players dont have to reposition away from barraged areas as you make it sound.
My point is that pios fulfill several roles, meaning they're not available to every single tank all over the map at all times, and having to produce a weak unit just for repairs is not an advantage. You have two ways of repairing instead of one, which is a good thing. Why do you think it is a doctrinal ability for OKW? I never commented on arty in regards with crew repairs, so I think you have misunderstood something with your last comment.