A 5.5-inch dual-purpose gun could arm cruisers and large destroyers

How useful would a 5.5-inch dual-purpose gun have been in the Second World War if development had continued?

Very useful: it could have equipped many light cruisers and possibly some large destroyers.

A wartime 5.5-inch dual-purpose gun could have been useful as the main battery of many light cruisers and perhaps the largest destroyers. The weapon would combine anti-surface fire with an anti-aircraft role. Maturing one intermediate-caliber system could therefore affect the armament of several ship types.