In DA2 he's one of the (few...) voices of reason. He was rather adorable (if you could ignore the psychotic) in DAO; he's rather devestating in DA2.
I can't help but think this is some sort of fan service. He WAS a fan favorite.
Of course, he might have been a dev favorite, too. ;P
His somewhat large role in DA2 does explain why Greg Ellis didn't do the voice of Anders for DA2, though. Guess it was easier to explain that Anders voice was different rather than Cullen's.
That and one might not think it was the same Cullen with a different voice actor. And they wanted us to question Anders' presence.