honestly, morde doesnt get any more damage then he starts with. (ap mordes are laughable) so the MR from QSS will make him hit like a wet noodle.
It doesnt make his ult completely useless as suggested however, because good mordes are familiar with people who get it, and will throw down their ult at the last second so you dont have time to use it.
AND please dont respond "lol u noob, just react quickly" sorry I do not have tenth of a second reflexes on items with actives, which just about everyone in the game tends to neglect using.
admit it, how many times have you forgot to use randuins, sword of the divine, or died with zhonya off cd?