About the GTA4 comparison, I wonder how the player would feel in a Halo game if he turned around and half the Grunts and the Wraith would disappear and a bunch of Flood with a Mongoose would spawn out of nowhere instead. I think random generated NPCs like that don't really work in an FPS game.
Seriously, comparing these games is stupid.
I know you are trying to make a point, but it is very unclear. Pick any open world game, say Red Faction Guerrilla. Lots of enemies, large destruction, vehicles, etc. all with good IQ and performance. Defending Bungie to the last breath is very irrational. We should expect more because we have seen it done in many games and it is no longer 2007 (not that is was a good excuse then).