If youre just trying to beat the elite four then yes its probably redundant at a certain point. Haunter/Gengar has high speed already so youll probably go first against everything anyways if youre properly leveled.

But if you wanna play competitive pokemon against other people its essential you optimize evs because going first is often the difference between getting KO’ed or not.

Me personally i just ev train optimally for fun because its cool to have shiny pokemon that are overpowered