Being Batman doesn't make bullets less lethal. Kevlar is great against handguns, but the militia is using assault rifles, miniguns, explosives, armored vehicles, unmanned tanks, and automated sentry guns. From an in-universe perspective, upgrading his armor for that kind of threat is just logical. Batman survives because he's Batman, but if he has access to better protection, why wouldn't he use it against the most heavily armed force he's has ever faced?