When you think about it...how many Sith masters have to die before they realize that maybe having an apprentice is a sure fire way of getting usurped/killed. I see apprentices as a liability, unless your goal is to eventually kill said apprentice and somehow absorb thier force power...let them struggle.
The apprentice killing the master was exactly the point. It not culled the weak from the order, but it ensured the apprentice would not become the master until he was in fact stronger than the master, making the order progressively stronger as time went on. That's how Sidious and Vader were able to wipe out the Jedi Order almost completely. And before you say that Vader actually almost died at the hands of a Jedi, keep in mind that he was still an apprentice and hadn't had time to learn the ways of the Dark Side before he was thrust into an accelerated time table for Order sixty-six.