by out4blood
I can't answer the general question of "superfluous" but I think I can for Terra Mystica.In Terra Mystica, the game fundamentally cannot work if all of the races are exactly same, as the whole point of the game is races all have unique terrain and unique terraforming processes. The game only works when they are different.
Now that we have established they need to be different, there is the question of "how different" should they be. It's possible they could be devoid of some of the special powers, and the differences in cost, and the mechanical gameplay would be no worse for wear, but it's likely it would greatly undermine the theme, since the special powers themanically link to their racial characteristics, e.g., engineers build bridges, dwarves tunnel, fairies fly, cultists have religious bonuses, etc.