My 2 cents.
Torps tend to be a Rebel ordinance and Missiles tend to be an Imperial ordinance. Yes, I understand that that there are exceptions, but this is generally true. That being said, Rebels tend to have higher base firepower than their Imperial counterparts so they less need the extra oomph that ordinance provides with the exception of Z's. Z's and most Imperial ships have a base attack value of 2 so the points spent on the extra attack power (even if it is a one-shot boost) seems more justified.
To put it another way, why would I spend valuable points to increase my X-Wing's attack value from 3 to 4 for one shot? Now it may be worth it to increase my TIE Bomber's crappy attack value of 2 to 4 or 5 for 1 shot if the points and secondary effects are worth it.