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.
It is much easier to win with a short bench when you have four HOF position players in their prime and two more position player starters who are All-Stars.