It's a waste of time to kill every mob. You're not getting anything from those mobs that you couldn't get from the ones on patrol and they aren't crucial to the story, so there's no advantage in wasting time. There's no sense in complaining about getting the more serious party members, who are doing things like the exotic weapon bounty and have probably done that strike over and over, because you're clearly a more casual player. Not that there's anything wrong with that. Some people like to just shoot and kill everything and aren't concerned with getting legendary and exotic gear and then there's the other people. If you don't like it, group up with other casuals and run the strike slowly and kill everything. There, problem solved.