1st: in other tbs if you lose your capital some times based on your the morale of your populous there would be a civil war of sorts were your most disenfranchised people would leave your control and spawn a new empire(s)

2nd: in civ 1 if you killed off a pc a another race would spawn in some random place away form you that was unclaimed(unexplored)

3rd: Kill the king!, I mean as a victory condition if your home planet falls so dose your tyranny...

true, if you kill of a race another one thats spawns would be at a disadvantage but look at minor races...