The Heavy Laser (“HL”) is an improvement in sheer power over the Light Laser. Destroyers do not have Rapid-Fire against it and that fact alone may make it better fodder than Light Lasers in certain cases. Large numbers of Heavy Lasers may force your opponents to use Bombers, which have less firepower than Destroyers and are not effective in fleet-against-fleet combat.
Note: For pros and cons of defensive structures, see Defense Comparison