Why would a ship NOT fire all of it's beams? It's a great improvemet, can't wait to try it all.
Exactly. It's a war. This is the way it would be in real life. You don't go easy on your enemies.
You're straining your logistics network if you fire everything haphazardly. You don't turn your AA guns on a surface combatant or your CIWS on the horizon except out of desperation; doubly so in Freespace where you're dealing with energy weapons on shared power grids and heatsinks. The Colly's imploding under the stress of her LRBGreens in High Noon, it doesn't make sense for her to worsen it for the superficial damage of lighter weapons.
Of course, all bets are off in Their Finest Hour since it's exactly that sort of desperation. If all you've got left to shoot the Yamato
at Samar is your pistol, then lock and load.The beams should probably be sequentially locked from back to front as the Colossus blows up, though, so it stops shooting from empty space.