Firearms are faster a killing; it's why we use them. However a military entering a hostile area doesn't just stroll through casually even if they could know for a fact no guns are in the area. They still have to contend with fertilizer bombs, molotov cocktails, suicide trucks, bows and arrows, those 3 watt laser pointers, plus any other improvised weapon. If lasers weren't being banned from war for cruelty would maybe even be more effective at taking combatants off the battlefield than an AR-15.
It's likely that insurgents wouldn't be treated as irregulars or partisans under the Geneva Convention in any case, so the incentives to not just "blind away" with lasers would be more about personal reservations, retaliation, and PR.
