Well it gets both previous characters out of the way but still makes them important as the parents of the new character. It also acknowledges their importance at the end of the game. More so in the LS ending , but I thought it was a nice touch.
Another advantage is everyone probably wants to know whats been going on (as a player) and the characters story is planned around exactly that uncovering why his or her parents were killed and why they were there in the first place. So the story is as much about self discovery and coming to terms with this heritage (and the factions that want to take advantage of it) as much as it about defeating some bad guy.
The character is also immune to the sort of malachor corruption effect (which would be hugely magnified at the heart of the Sith empire if you think about it).
But they have inherited a variation on the force bond which protects them and the people they come in contact with. You can still go DS , but it will be on your terms