Think about it, if Revan and Bastila hav a kid, they get an emotional attachment to it.
If Revan or Bastila, or the kid for that matter, is killed by say, a sith. The survivors out of the 3, will most likely seek vengeance, which will evetually lead to the darkside.
If u hav any of those 3 darkside, especially Revan, it wont be good for the jedi will it.