You need to realize the forward arc is just the OPTIMAL firing solution for the custodian, it isn't the ONLY direction it can engage from. Yes it will suffer, but it also has much greater benefit when it is not crippled. If it moved 15cm you might have a point, but you are trying to tell me the performance of the ship in the crippled state with a BM outweighs the tactical capability in speed, maneuverability, and durability it would have to position itself in such a way that crippling it would be far more difficult to accomplish in the first place.
The fact remains that although the ship wouldn't perform as well once crippled and still under fire (no surprise there, I mean you could almost say such a situation would be.... crippling), it will perform MUCH better in all other circumstances.