The bombardment rules specifically prohibit ships from firing on planetary and vessel targets in the same round, so the Primus would be firing lasers at the planet no matter what. If it's tactically safe for it to do that, it's a moot point, because (now, with the Armageddon rewrite) it has all the time it needs anyway once the defending fleet is toast.
Between Battle lasers and Matter Cannon, and on the Dilgar side Bolters and Disruptor Torpedoes, the species with mass drivers actually have the most effective existing planetary attack power anyway and need them the least. This is slightly bizarre, but what the heck, it saves time.
As a matter of interest, an Octurion with twin mass drivers could be expected to take out forty- eight emplacements in the first round of bombardment fire. Fourteen of those are Beam dependent, could be more could be less- but that's enough to strip the fixed defences in anything less than a War level scenario, and come close even then.
Without them, twenty- four a round.