The thing is... if you actually plan to use a nine-man bullpen (or and eight-man 'pen), and carry only four bench players, you pretty much have to use multiple pitchers in single innings. If you go to the bullpen after five innings (or, God help us, sooner), you're going to need multiple pinch hitters - you may only need one or two, if your offense doesn't mount any serious threat in the last four-five innings. But if the offense actually gets something going, you're going to run out pinch hitters; you're going to have relief pitchers coming to the plate. That's in nine-inning games. You get into a twelve-inning game, where you've been cycling relievers (and using pinch-hitters) since the sixth, you're up the proverbial creek with Hoby Milner at the plate. Only a matter of time.
Bottom line: If managers are going to try to use six pitchers a night, or more, rosters have to expand. OTOH, if MLB is serious about game length, then they have to limit roster size; it's the only real limit on how much managers can abuse - and lengthen - the game with constant personnel changes.