It's ultimately more efficient to run more strikes and kill less stuff in them due to the rewards being heavily loaded in the end. Yes, you can get a legendary engram off trashmobs in a strike, but you're still better off going for completions for the reward at the end. I will however stop to kill stuff if it helps me complete something, which would be the grimoire. However, since I'm mostly done with grimoire stuff, that means that unless it's an ogre or a hydra and I don't have to kill it, I won't kill it. The true skill in running instances or strikes in MMOs lies in how efficiently you can do it. Back when I played WoW this basically meant that you'd skip everything you could and every time you could pull multiple packs of mobs simultaneously to make it faster, you would.