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?