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. . . I think is dictated as much by simple arithmetic as anything else. Pinch-hitters succeed how often? Most years pinch hitters hit for about a .225 average. I couldn't find data on OBP but just assume it too is at scrub level. Given that, pinch-hitting doesn't seem to be a great route to success. If you platoon your relief pitchers instead of your pinch hitters, seems like you're optimizing your chances of succeeding.
Don't get me wrong - I hate the bloated bullpens. I think they're basically a huge security blanket for teams, but the reasons behind them seem to make some sense.