well, if the parasitic nature was what completely governed the sith, they would not have been destroyed in RotJ. it was the good and compassion in vader that broke the cycle, not his yearning for power.
well, thats a contradicting statement. accoring to the sith rule, it was a fine plan.
all possible outcomes:
1-luke turns and kills vader-becomes palps' apprentice
2-luke turns and kills palps-becomes vader's apprentice
3-luke doesnt turn, emporer and/or vader kill him, same as it ever was
either way, two remain. palps would not have killed vader, and vader would not have killed palps. it was the good in vader that ended the sith.
first by killing the master and then by sacrificing himself, or rather giving up his interest in power and once again becoming anakin. even if he DID kill palps he would have instinctively found an apprentice.
:edit: and when you think about it, the rule of two is not difficult for them to follow, since it ensures that the master will never have to share power. so there really was no danger of that being broken.
In a way I agree... the rule of two is some kind of insurance the Sith can endure... it is the only way. But it is fragile... apparantly it only works when there are two, but the relationship is always tense since the basis is hate, anger jealousy etc... So the apprentice will always want what the master has and the master is afraid to lose his position. So an apprentice cannot get too powerful for a master, and for the apprentice there's always looking for that one moment of weakness...
Of course it is a good rule, it was the only way for them to survive!
But you seem to have missed the obvious here. It meant that there were only two of them! Which rather obviously makes it easy for them to be wiped out forever. Which is what happened. Vader suddenly going all soft wouldn't have made much of an impact if there had been dozens of Sidious types.
It is the Dark Side's basic nature that makes them so vulnerable. If there only ever being two of them is actually the best position they can get, then the weakness of the Dark Side is exposed by it.
__________________
"We've got maybe seconds before Darth Rosenberg grinds everybody into Jawa burgers and not one of you buds has the midi-chlorians to stop her!"