For reb's bench, I wonder if they'd have more motive to carry a 5-man bench is they had 5 subs worth carrying? If the extras were more interesting than a Baxter or Herrera or Szczur, maybe there'd be more interest. If you have Olt up and he's hitting .180 and whiffing every time with 1 HR per month, not much value.....
Okay, geez, Big Red Machine of 1975 had primary middle infield backup Darrel Chaney, who hit .219 (yes, he played more than you think--175 PAs); primary backup OFer Merv Rettenmund, who slugged .314; back-up catcher Bill Plummer (189 PAs), batted .182; and Ed "no, I didn't obstruct Carlton Fisk" Ambrister backing up OFer, who batted .185. (Okay, they had Dan Driessen). More guys of no particular note.
Point is that it's useful to have a long bench, even if that season they're not so hot.
By the way, 1975 Reds used total of 12 pitchers the entire season, one of whom threw 2.0 innings. Went with a 4-man bullpen almost all season: Eastwick, McEnaney, Clay Carroll, Borbon. Nobody else had more than 8 relief appearances that season.