1) You need more than two QBs to practice with.
2) WRs benefit from having a QB who knows the offense and can help them learn how to read defenses correctly.
3) The Defense benefits from competing against an experienced QB who presents a more challenging opponent.
Yes, it is true that Stewart's reps take opportunities away from Hunter Moore to practice and improve. However, the benefit to the rest of the team easily surpasses the cost to Hunter.