Subs are pretty uber. Even on defense, they cost less per HP than any other unit, and their first strike ability only amplifies that. They are most vulnerable to Fighters, but their effectiveness against Carriers offsets that.
For 24 IPCs
4 Subs: 8 Attack, 4 Defense, 4 HP
3 Destroyers: 6 Attack, 6 Defense, 3 HP
Even when attacking, Destroyers will only win that battle 55% of the time, but on defense, they win a mere 18% of the time.