I'd use it, be it longbow or crossbow. But it has to shoot super futuristic bodkin-style armor piercing arrows, And it can't make a noise like a rifle does, meaning a stealthy Guardian can get in and out of a fortification alone without too much trouble. The glorious thing about bows compared to rifles, is that it can fire in arcs, it doesn't need a direct line of sight to hit targets. Bungie could exploit this in-game and use it as a tactical change-up in gun fights, sure it won't do as much damage, but if you're hiding behind something, or on a reverse slope, the enemy is screwed.