It isn't ***, precisely, it's half damage. Half of 4000 is 2000, which is often still worth doing. Apparently it works for C-Station. Though possibly it would work better to hit the shields with energy and the ship with the torpedo...I'd have to try it. I often use assault guns on shields just to make them overload faster than my kinetics alone can do.
If you're using a Capital Ship or Cruiser that gets multiple torpedos, then yeah, I suppose. I should've been more clear.
I'm talking Destroyer and Frigate-level stuff. Where you only get one Reaper or 2 Atropos per missile slot for the ENTIRE BATTLE. Sure, you can do 2000 damage against shields and maybe overload a Frigate or shield-taxed Destroyer.
Or you could time it for when the shields are down and do the full 4,000 damage. I prefer to use them to their maximum potential. Saving them for an Overload or when the enemy turns it's shields away from you tends to work better for me. Half a dozen of those things getting past the shields will cripple even a Paragon or Onslaught and a pair of them launched together can 1-shot most things smaller than a Cruiser
Anyways...this was for tips...so lets see...
If you ever see the AI launch Harpoons (Missiles with a Blue exhaust. You'll learn to recognize them FAST), immediately stop firing and prepare to shield. They usually launch them when they think you'll be stupid and keep firing, which would keep your flux high enough for them to overload you can kill you with the Harpoons. This is probably THE #1 way newbies die in the game.
Pressing X will Hold Fire on all of your weapons, stopping all flux buildup except from damage to your shields. Take the Harpoon hits to the shield, THEN resume firing. You CANNOT tank Harpoon hits to the hull.