1. Torpedoes still have their own cooldown timers.
Say you fit 4 launchers to a T5 escort or cruiser. You fire them all at once and 4 torpedos hit your target.You still have to wait 6-9 seconds depending on torpedo type before you can fire anything from your forward arc again. By then your target may have moved out of your firing arc and/or turned a fully charged shield arc towards you. What you'd be capable of is a massive alpha strike and not much else.
2. You'd gimp damage versaility by fitting all or mostly torps.
Fitting a large number of torps makes you vulnerable on your side arcs since your broadside capability will be much weaker. You might be able to bring down a single shield face and do some hull damage if you fit 4 quantums, but then you'd be hitting your target with gimped dps if you turned to shoot with your rear arcs. You would still have a good reason to fit energy weapons in both arcs for sustained dps. If you're flying an escort, you'll still get much more dps from dual cannons and alpha from dual heavy cannons, and don't forget the tactical boff abilities for cannons. Variety will always be your friend.
3. You'll still want the special benefits of energy weapons.
Phasers, Disruptors, Polarons etc would never become obsolete. The more you have, the greater chance you'll hit with their special debuffs.
4. Fitting 4 tricobalt launchers would be awesome for the opening volley on an unshielded target, then you'd be a sitting duck for 30 seconds.
Sure you could do this, but tricobalts can still be shot down, leaving you with potentially zero damage capability.
Let's hear some reasons why shared torp cooldowns are a good thing?
I don't really pay much attention to numbers, but I remember my photon X with moderate skill and no torpedo damage tactical consoles dealing about 5000 per hit.
So we can assume that Quantum X with full skills and tactical consoles deal 7000 or more per hit.
High Yield 3 gives about 15% more damage than High Yield I, which deals 200% damage. 200x 1.15 = 230% or something like that.
A cloaked bird of prey approaches you, he activates High Yield 3 and lets it wait until the global cooldown for High Yield dissapears. And then he then decloaks, activates Attack Pattern Alpha III, Attack Pattern Omega III, fires High Yield III, followed by another High Yield III and his other 2 torpedo launchers (because 4 front weapons)
(7000 x 2.3 x 2) + (7000 x 2) = 46200
I'm not sure how the bonuses are applied in-game, however the tooltip for my Attack Pattern Alpha III says +60% damage and the tooltip for my Attack Pattern Omega III says +50% damage.
Feds can use mask energy signature, or they don't even need to be cloaked at all, they just need to have pre-activated high yield III 20 seconds earlier. Range and Power doesn't matter. They can load it, approach you and then just nudge 9.9km, launch it and then maneuver out of range. Once launched it's an unavoidable blob of burst damage that will hit you sooner or later.
Oh and watch out for Fire on my Mark III, tactical officers tend to have that. I don't know what kind of foul sorcery they do but I do know from seeing the in game numbers that AP alpha, AP omega and fire on my mark III has been able to inflate the damage of my photon torps from a puny 5000 to a quite impressive 13000.
Any one who uses multiple torpedo launchers of the same type is stupid. 1 they have different cooldown times. 2 They have different speeds. If you bother to work things out you can have 2 different torps hitting at the same time with a third fired as they hit. First torp fired with high yeold 3 rd fired with spread. Then add in effects suach as Alpha Strike and Fire on my Mark. Then 3 consoles with a total of +64 torp damage and you are doing significant damage.