By wasting ammo and time on useless mobs you give yourself a chance to get greens, maybe a blue, and if you're having an extremely lucky day, a single legendary. At the end of the strike you will receive Vanguard Marks, Reputation, and at LEAST a few blues if not a Legendary. So, yes you can spend your time thoroughly killing any of the mobs that will undoubtedly spawn in as soon as you leave using time and ammo (which you will need to actually progress within the strike) but you're going to make it harder for yourself later on. Whether you do it or not, you'll still receive the completion rewards. By all means, go for it. But for someone like me who does it for completion (and frankly, because it's fun) don't complain when we do what we do. If you get some Legendaries or something along the way, more power to you.