To me it is a question of balance in order for the game to function you need balance traditionally detectives are put in to allow the town to have some form of power roles and in many games they are to mind essential any high level conversion game needs detectives otherwise you get to the point once you have lynched all original scum that traditional methods of scum detecting do not work. However I broadly agree and that is why in the recent games I hosted with my Roman theme had no detectives at all and the only game I hosted with detectives recently was Departed Dethy Mafia which needed detectives for obvious reasons. However I think detectives can work and can make the game better they just need a balancing mechanic that makes there use somewhat more fraught with difficulties.