From my understanding, arrow shifts the area of influence. Putting blue arrows on a DC would make them stay closer to the goal, which although likely stop any runs or crossing, would risk getting pounded by mid-range shots. To build a effective defense you need to balance your area of influence somehow.
There are many formations that have balanced configuation that does not require any arrow, such as vanilla 442 and 352, but some formations may have inherent requirement of arrows for best effect. The effectiveness of a formation is greatly reflected on the "chance-creation ratio", where a good defensive setup actually prevent the opponent from getting chances at the goal in the first place.
If you are constantly being "attacked" without any player getting bad rating, often it is because the defensive setup being critically flawed.
Putting a lot of arrows on a formation without careful planning is more likely creates flaws at the defense rather than strengthen it.