Destroying a capital ship like a Mon Calamari cruiser still took a great deal of firepower from a similar ship, such as a star destroyer. And in order to unleash its full power on the Mon Cal cruiser meant getting close enough for return fire with damaged/destroyed ships and loss of life. It seems logical to use a weapon that could easily overwhelm the opponent's shields and destroy the ship without incurring any loss on your own part. The superlaser could easily be dialed down from its full planet-destroying level and recharged quickly enough to take out several ships. It's a smart tactical move: pin your opponent against a wall (the Imperial Fleet in this case) and use your overpowering weapon against them. It then is simply a shooting gallery. And yes, they are all on "the forest moon of Endor," so what else would they shoot?