TBH I think this is kind of a misconception due to the way they wrote the guide:
From Player's Guide - "At the big league level, you'll need a bench coach, hitting coach, pitching coach, bullpen coach, 1B & 3B coach. The 1B & 3B coaches contribute to the development of players' basestealing and baserunning. The hitting coach, with a little help from the 1B & 3B coach, contributes to the development of all hitting ratings. The pitching coach, with some help from the bullpen coach, contributes to the development of pitcher ratings. The bench coach takes over as manager after any managerial ejection."
I can see how people would get how the Bench coach only contributes if the head coach gets ejected, etc.
From FAQ:
| Does a major league bench coach have any effect outside of when the manager is ejected? |
Bench coaches don't just have an impact when they take over for the manager (i.e. you). They impact all of the ratings and performance of the players.<br> <br> The main ratings that the bench coach has an effect on for position players are hitting, pitch calling, and base-running. For pitchers at the minor league level they take over (to a lesser extent) the bullpen coaches role.<br> <br> At the minor league level bench coaches play a role in all facets of player development. That is why they are one of the most desired and highly compensated components of your coaching staff. |
This i think is commonly overlooked. Spells out bench coach directly has impact on performance.