Placed near gates they hit entire waves with their first volleys. I wouldnt just unlock them, but if you need that extra push countering waves and other big attacks, they sure help.
In my last game, 4 caps of heavy beam turrets -vs- ~½ mk1 cap lightning => killratio 20-1 (cap vs cap that would probably turn out to be somewhere around 5-1).
They are a very good support for the fleet, but need something else to actually finish of the individual targets.
When you get units stuck near the gate, on tractorbeams, you can hurt 1000s of ships with volleys of just lightning turrets, thats quite impressive overall damage (if there isnt some cap in play).