Frigate ASW and destroyer air-defence roles

What should a frigate's primary role be when requirements keep turning it into a destroyer?

The proposed division gives frigates anti-submarine warfare and destroyers area air defence, with secondary capabilities retained in both.

The US Navy's loss of cruisers and frigates has concentrated several roles in its Arleigh Burke destroyers. Sal Mercogliano argues that the Burkes are imperfect submarine hunters and that LCS's high-speed jet-drive design is poorly suited to that mission. His preferred frigate would concentrate on ASW. The complementary destroyer would carry the large, elevated sensors needed for area air defence. Both ships would retain secondary capabilities, while a primary role would set the design priorities and limit competing requirements.