A major use of Decoy's would be to counter the one large Box launcher salvo. If you are taking multiple salvo's with small numbers of leakers then a decoy is probably inefficient, if you are taking one salco which intended to kill or cripple your ships then being able to decoy half the leakers will probably turn kills into armour and shield damage, so requiring a much larger salvo to achieve the goal
To expand on this: if you have, say, nine decoys on a ship, and you face a box launcher attack, you can launch all nine decoys and avoid 90% of the missiles, basically making the box launcher attack toothless. In contrast, if the enemy is using reloadable launchers with several salvos, you would only launch one or two decoys per salvo and avoid half or 2/3 of the incoming missiles. Of course, reloadable launcher waves are smaller and thus vulnerable to regular point defense envelopes, but presumably there is a tonnage tradeoff between using decoys versus conventional PD methods, and each is better against a different kind of enemy.
I think this in turn will increase the importance of intelligence, scouting, and building a diverse fleet so you can adjust fleet composition to meet new enemies, which is a very good thing.