Cassandra states why she was looking for Hawke in DA2: to serve as a voice of reason to the side they supported at the end of the game.
Which is kind of a shame- I honestly think DA2 would have worked better on some levels had Hawke died as a martyr for the mage revolution if they sided with the mages against the Annullment. It could have played well into the 'no one knows what the Champion actually was' idea that Varric and Cassandra paly around: that no matter what Hawke actually believed, that the martyr persona would have been twisted to suit the purposes and needs of others.
Plus, I appreciate the kick to the guts that it would have given much of the fanbase. Live, and die, by your morality in the face of assured defeat? How lame! Heroes don't die for their cause!
But the obvious drawbacks are obvious. Can't have the Champion serve as the voice of reason if they're already dead, after all. Cassandra's investigation loses much of its relevance afterwards.