A Darth is a Darth

Well it's not that they are going to do anything, but the Emperor wants everyone to believe he is extremely weak and powerless. That way if Vader isn't around and someone attempts to start some sh*t, he can dispatch them with ease.

What I always thought was in each Episode a Sith is revealed and they last till the next Episode.
So in EpIII, Maul - Vader - Tyranus (Dooku) battle out to be Sidious's apprentice.
I think that would have been kick arse.