Minutes is always more important than starting or not. Finishing is more important than starting as well.
One reason why I could see bringing Upshaw in off the bench is that you get some secondary scoring where Upshaw's first minutes in the game are after an opposing big has already got 4-5 minutes of work in the game or against a backup where Upshaw will have an even bigger skill advantage against. Given our struggles to score, it's a good way to find opportunities to get Upshaw into the game quickly with some easy looks.
Dorsey's not playing because of defense. While we desperately need his shooting ability, he's a true frosh who quite frankly doesn't play enough defense at this point. Given that this team is built off of playing strong defense, rebounding, and scoring just enough points, Dorsey's just not pulling on the same rope at this point. Not getting playing time will also get his attention in that if he wants to play, he's got to focus on playing defense.