I think that both are backups, at best, given the other options we have in the outfield. There is a place for role players. I don't agree that Stewart is "better." He's a better hitter. He's a worse defender. He's a worse baserunner. Depends on what flavor of back up player you want. If you want your backup outfielder to also DH, Stewart is the better option. If you want plus defense it's Mullins.
Finally, point. Mancini is a better player than Valaika. But they have different roles. Mancini could never backup at 2nd, SS and 3rd. You need to have someone that can do what Valaika does. You might want someone that can do it better than Valaika, but that person isn't Mancini.
Been saying this for a while. I think the Os have a lot of guys that could be interesting openers. I don’t know how many of them can go 5-6 innings but definitely agree that they have 2-4 inning guys galore.