The have an overall negative effect on the value of the game, I don't think many would argue that the things they have brought into the game past 15 years has devalued it.
But obviously some coaches have good winning records other crap records, it is mainly about the quality of players but a good coach can help a club win. They all seem to leave their clubs weak when they leave though like Bennett so even then the lasting value to the club is questionable.
You would get the same under a captain coach system or part time coaches or big regulations on coaching.
Main point is they are passengers on game day, so do something useful.