Why would Lorca want Saru as his XO? Lorca is a guy who wants to win at any cost. How does having a pacifist who freaks and wants to retreat every time his spidey-sense twitches serve that purpose?
The Discovery is, first and foremost, a science ship. Having an XO who is a former science officer is an obvious decision. Having an XO who is a former science officer and can instinctively sense the presence of danger even before it becomes apparent is an excellent decision.