Seriously, why the ever living f*ck do people insist on going to knife fighting range during space combat?!? This is not naval combat during the age of sail! If you can se the enemy with your naked eye the captain has done something wrong.
The spaceships in question mostly have one main gun for attacking enemy ships and clusters of CIWS for dealing with torpedoes and small craft, it does not have an age of sail broadside! Since the main gun is relatively slow firing you would want to stay at range so as to give you time to maneuver between shots, both to dodge incoming fire and to line up your own shot. This strategy is the most likely to let you survive the encounter and unless you are a dreadnought attacking a light frigate is the one that should be followed.
The purpose of torpedoes in this sort of fight is to harass the enemy and maybe just maybe let you inflict some serious damage without using the main gun. Usually the CIWS should handle incoming torpedoes fairly well and the chances of getting a hit is small, this is mostly a measure of processing power since the ship can dedicate far more cycles to targeting the torpedoes than the torpedoes can dedicate to evading (and still stay on course). This can be changed if you either saturate the point defence by targeting one ship with a crap load of torpedoes or by having a few torpedoes exchange their warheads for ECM and ECCM packages so as to degrade point defence accuracy.
For a torpedo to have any chance at all of scoring a hit they need to be fast and agile so as to give the least time and chance for point defence to engage them. You Do Not under any circumstances put something in the casing that degrades either speed or agility. A ECM or ECCM package that fits in the warhead section and doesn't degrade performance is a good thing but the way the torpedoes in Mass Effect are described they sacrificed speed for a really stupid reason, the torpedoes were designed to mimic ships...??? Why???
Decoys that mimic the ship should be kept near the ship during combat so as to confuse targeting, if the enemy has to split its fire three ways because you've confused them with decoys then that is you taking a third of the damage you would have. Also keeping the decoys close lets you network them into your ships ECM and ECCM system so that your bridge crew can use not just pre recorded patterns but actually change what the decoys show as the battlespace changes. Keeping them close might also let you help keep them alive a little longer by shielding them under your point defence bubble, these things would not be cheap so you'd want to keep them alive for as long as possible.
TL;DR Mass Effect torpedoes are stupid because they sacrifice speed for a dubious try at being shitty decoys.