From a post to DAVIDWEBER.NET forums on 6/20/2011

Shipboard beam rates of fire limitations

    Both lasers and grasers are pulse-fired weapons and rate of fire is dependent primarily on charge rates (there are secondary factors, but they are relatively less important). Grasers are favored over lasers for anitship weapons primarily because they are effectively longer ranged are more destructive, but also because they can be fired at reduced charge levels by using less of the plasma available at any moment, which also permits them to be fired more rapidly. The grasers' on-mount plasma capacitors can also be pumped while lower powered shots are being fired, keeping them "topped up" for rapid follow up shots (this, BTW, is one reason the Havenites were able to use their broadside weapons in a point-defense role, although the RoF is still a bunch lower than the RoF for a laser cluster). Point defense lasers are mounted in clusters because each emitter has a fixed rate of fire; multiple emitters allow them to fire far more rapidly on a per-mount basis. In addition, the point defense laser cluster gives a far better rate of fire for a given mass and volume penalty than a graser would, and the laser's effective range is fully adequate for its role. When Terekhov used his point defense clusters in "force suppression" mode in Shadow of Saganami, the lasers fired at full power (the reason they ripped the freighter to shreds). They were used because even at minimum power the grasers would have completely destroyed the target and because they could fire almost instantly from "cold" status, which the grasers could not have. The rapidity with which the clusters could fire also made them the weapon of choice for such close range, shoot-from-the-hip circumstances. Grav-lens energy weapons are gravitically/electronically [steered -Ed.] (depending on how one wants to think of it), allowing them to switch targets very rapidly (not instantly, but well within the recharging "window" of even relatively low-powered graser shots. The emitters are also capable of physical changes in train, and the window in the sidewall moves with the emitter, but the process is much, much slower. It does allow the broadside weapons a much wider angle of train than would otherwise be possible, however.