Separate anti-aircraft batteries or dual-purpose guns?

Why keep a separate medium anti-aircraft battery when dual-purpose secondary guns already work?

Specialist guns can perform each role better, while more numerous dual-purpose guns simplify logistics and training and provide redundancy.

Separate batteries let designers optimise each gun for one role. A ship can therefore carry a few weapons approaching a complete solution to one task, or more weapons that handle several tasks adequately. The preference here is for more dual-purpose guns because common weapons simplify logistics and training. They also provide redundancy, reducing the risk of losing all useful defence on one side of a ship.