If you don't have siege starships or heavy beam cannons or something,t hen all slowing down raid starships does is make for more fireworks. You need something that actually hurts them before slowing them down is of any value. You arn't going to stop them reaching your forcefields if nothing you have goes through the armor, no matter how slow you make them. :p
That is true. Slowing doesn't equal damage.
That way it would indeed be pointless.
But slowing does increase the total damage that anything that does damage to the starships can do before it's too late, so combined with firepower, slowing is a very cheap way to increase defense strength. Unlocking higher mark Heavy Beams and starships gets really expensive compared to a few grav turrets, which can allow all turrets and ships in range to fire several additional times at the very least.
