As I look back over this thread, I think there is a bit of a disconnect. I think "dozens of people saying that their attempts to use proton torpedoes have not worked well" probably refers to single torpedo usage, and not to them setting up the two-shot attack that is the key to Scottie's whole argument. As has been mentioned, a single torpedo attack is probably not going to get a kill (though it is statistically possible) but ganging up for multiple shots on a single target raises the odds exponentially. It is certainly very thematic, since the Rebels had to have multiple torpedo attacks to take down that first Death Star…
Torpedoes are certainly not the one-shot-one-kill weapon that Cluster or Concussion Missiles can be, but when used as Scottie proposes they can almost guarantee a kill. The question then becomes whether that 8 points is worth it.
I guess all my assumptions are based on Rebel against Imperial. In mirror matches, the torps become less effective, probably.
On the contrary, Torpedoes are far more useful and powerful in a Rebel mirror match.
Rebels have, generally, less evade dice and no access to the evade action, meaning that you generally get more hits. It's still not a one shot kill, because of the shields, but can be pretty devestating.
Especially when Wedge fires one at a Y-Wing.