I race through them because I want my damned marks, and to get to the reward screen faster. I've figure out speed-run paths, so that's what I take. Rushing means I get the missions done with faster, which I can get more missions done faster, which nets me more engrams faster. I don't rely on strikes for legendaries, I rely on the Cryptarch's leveling-up mail. I've been quite lucky with Mr. Crypt and his mail.