You couldn't be more wrong. If plasma torpedoes hit then their effect would possibly be better than proton torpedoes assuming that the ship has any shields remaining. That is quite a few asterisks to make it better than proton torpedoes. The plasma torpedoes has a higher probability of taking of 4 shields than the proton torpedoes but how many ships have 4 shields again? If you do 3 hits and it has well 2 shields then congratulations you have done equivalent of 3 face down damage but paid for 4 damage in squadron points.
The thing that really kills plasma torpedoes is the same thing that kills ion torpedoes. It has to hit and has no dice modification abilities. In a matter of fact it takes away dice modifications abilities which is one of the many reasons why missiles and torpedo upgrades are so underwhelming to begin with. Turning 1 focus result into a crit is not a significantly substitute to re-rolling any or all of your dice but it is better than what plasma torpedoes have. Getting that one extra hit over the evades, even if it just takes off a shield token is far better than watching a good green roll canceling your attack all together, and with the new defensive abilities a decent green roll can cancel an attack that has been rolled even a 3 hit attack.
Face it plasma torpedoes are too situational to be of any good. The best torpedo secondary weapon is well flechette torpedo because of its point value and its effect goes off even if it does not hit.
I understand your logic, but disagree with your conclusions.
For one thing, the Plasma is only 3 pts, but lets you roll 4 dice. If you have some way of passing an action to the ordnance owner, you end up ahead. That alone is worth it. Forget the extra damage that would hit a shield. It's 4 dice that does damage for 3 pts.
Think of the extra shield damage as the eyeball that turns into a crit. Yes, you can't modify the dice, which means you would do one less damage on average. If you get one hit through, it turns into 2 hits...if they have a shield. So....situationally, you have a built in damage modifier.