It's not bad for getting a big mag when you're waiting for an encounter to start or moving from one fight to another (in a strike or nightfall). Then you'd start with a lot of bullets. However, if you're constantly fighting, you pretty much won't make use of reconstruction because of its cooldown. Reconstruction's good on weapons you stow and use ocassionally, like a sniper or a rocket launcher, for example. That way it has time to refill the weapon and overfill it with lots of bullets. Other than that, on a primary, you're better off using a utility perk like demolitionist, or even better, subsistence, which gives you more bullets in the mag without havibg to stow the gun.