The reason that this decision is not common is because (at least, if you handle it poorly) it destroys the game. There has to be a reasonable challenge there for you after you make that choice, otherwise your decision to join with the bad guy is just a non-standard game over.
In order to truly let you join the side of evil, therefore, the factions have to be evenly matched. At that point, the big bad can't be on the verge of victory, and the decision can't be as easily "rule the world at my side" as you seem to want it to be.
In order to truly let you join the side of evil, therefore, the factions have to be evenly matched. At that point, the big bad can't be on the verge of victory, and the decision can't be as easily "rule the world at my side" as you seem to want it to be.