Those white pulses were Captain Robau's first choice for meeting the Nero threat, though. George Kirk in the second exchange of fire added the red beams, which had more success in the task of defending the Kelvin
and later also the shuttles.
We might speculate that the pulses are an old-fashioned CIWS system, perhaps a successor of the similar looking "plasma gun" peashooters we saw on Archer's ship in "Broken Bow" and later on various civilian ENT vessels. Until the Narada
encounter, these would have been good enough for dealing with incoming sublight projectiles, so Robau would base his defensive stance on these weapons. But Nero's missiles would be tougher and more maneuverable than the average projectile, so Kirk would have to start using weapons theoretically more suited for antiship use.
Sure, these might be offensive weapons, but Robau would have little reason to use such - and Kirk didn't seem to have time to spare for firing at Nero.