Corvettes' Rail Cannon modules counter Fighters, and have the additional benefit of infinite range, despite having essentially the same DPS as the other light module, the Laser Cannon.
You can use Corvette's shield modules to cover a bunch of other ships, with no DPS penalty. This is most useful when it can annul enemies' hull bonuses, forcing enemies to inflict base damage rather than bonus damage.
You'll want to look after your Corvettes, as they have the highest cap metal cost of all bonus ships (with the possible exception of those from the latest expansion, which I haven't checked).
Edit: By the way, does anyone know what marks of Sniper Turret unlock which marks of Rail Cannon module, so I can update the Wiki? Is it Sniper II for module II and III, and Sniper III for module IV and V? Rain Cannon Mark N requires Sniper Turret Mark N for N=I through IV, though I haven't checked V.