[{quoted}](name=rG Thundrag,realm=NA,application-id=3ErqAdtq,discussion-id=NO4vTZy6,comment-id=0001,timestamp=2015-06-02T13:16:36.796+0000)
While riven's shield is frustrating at times it exists as a form of sustain for her since her base health regen is some of the lowest in the game. It is also the only thing that lets her even do her damage without instantly dying in team fights.
While Yasuo could be comparable to Riven as an excuse to not give her MR.
You have to take into account Yasuo has a multitude of auto attack sources an AA spell Q + reset which makes him life steal quite a bit during the laning phase and late game along with an inbuilt shield that is automatically activated.
If we're going to talk about lifesteal, Riven uses it well too, she even historically has been building 2 lifesteal items as her main popular build. She will out sustain a Yasuo.