In the 60's and 70's your average pitching staff was 9 or 10 pitchers (always a 4-man starting rotation), so you always had at least 7 players on your bench. I was looking back at some old Sporting Newses and found an article about Tony LaRussa's Chicago White Sox in about '79 or '80. At that time he was contemplating an 8-man pitching staff - that's right 5 fewer pitchers than the Twins are currently carrying. All 8 pitchers had the ability to start if needed. In that case, he would have had a bench of 9 players. In those days, platooning players was a regular occurance. You could pinch hit more, have defensive specialists - and always 3 catchers.
When the owners and players were beginning negotiations for the new CBA, I put together a proposal which included a salary cap (which I placed at $225 MM initially.) I proposed expanding to 32 teams with four 4-team divisions in each league. Playoffs would not include wild cards, but the 4 division winners in each league. The other option in my proposal would be expanding the rosters to 28 and 45. Thereby adding additional players to the major leagues. (I know that talent is already watered down and this would cause more players who aren't ready to be in the majors.) My twist to the 28 man roster would be that 3 players each game would be designated as an emergency squad. Before each game the manager would designate which 3 players could be used under emergencies (i.e multiple injuries in the game) and only after the 8th inning. The stipulations would include that starting pitchers could not be designated to the emergency squad. I would include five categories to chose from; infielders, outfielders, catchers, utility players, and relief pitchers. You could only designate one player from each category, but always 3 players.
In this day and age, with 12 to 13 pitchers and a designated hitter, it can leave a mighty thin bench on a regular basis. For the Twins, who regularly it seems are playing even shorter because of Mauer injuries or overworked relief pitchers, it would allow for some players to rest a day or two a week without losing the full 25-man roster.