It's too bad it's not more like EA Access, where you'd maybe get a % off new Microsoft Studios titles / DLC or early access or something. Right now it seems just like a backlog of games that are good for people who maybe haven't had an Xbox before but not for people already bought into the ecosystem.
Their site specifically lists discounts.
Discounts on Xbox One games
Love an Xbox One game and want to own it? Interested in an add-on or Season Pass for a title youre playing in the Xbox Game Pass catalog? Save 20% on Xbox One game purchases and 10% on all related add-ons, but only while the base game is currently in the catalog.
Unless that's not what you meant.