Yes to being worth building.

The whole point of the navy is to be able to control waterways. The whole point of being able to control waterways is to be able to economically ship large amounts of material and people; in the case of warfare, soldiers, bullets, food, water, fuel, etc.

An unmanned fast attack sub is going to be useless for defending your logistics fleet from strike fighters and anti ship missles. Even a dingy that has a guy in it with a rocket propelled grenade can send a cargo ship to it's grave. You have to have a surface ships with powerful defenses to protect them.