In AR the user needs to be able to tell which objects are real (R) and which are virtual/injected (A), but the latter type doesn’t need to be indicated by transparency. Consider the scenario where, instead of conventional HUD-type AR, we could conjure up the A elements as physical objects into thin air by magic. There is no particular reason why those would have to be translucent. Sure, depending on the situation it can be useful to control their opacity in order to be able to see what’s behind them, but otherwise there is no more reason than for real physical objects.