That's always been a point of serious ambivalence with me. I agree with the point of not grinding XP in a carefully-designed game, but sometimes it's cool to revisit old areas and not find them forever devoid of life and interest.
Personally I'm all for "adding new unique content to replace enemies killed" as seeing the game world change as consequence of your actions adds a sense of place, progress and achievement.
On the other hand I'm fiercely against any form of respawn and any mechanic that could encourage or reward grinding.
I think they are (and should be treated as) two entirely different things.
