Flechette is always useful since you can stress ships and it deals dmg.
Tractor Beam doesn't work as well on big ships (0 agility doesn't care the least).
I know the shenanigans with tractor is awesome, but I personally prefer a sure thing if you apply it to a TIE/D.
There are 2 ships that have 0 Agility off the top of my head... I think I would rather take my chances running into a Decimator or a VCX and have the Tractor Beam be useless against them, than not have the Tractor Beam and it be awesome against so many more ships. Flechette only works if you don't have a stress, so does not work against Dengaroo, PTL aces etc etc etc. Reduces things like the Falcon to 0 Agi, making C3=P0 useless. The TB has it in spades over the Flechette.