Ultimately, from a narrative standpoint, I agree with you. From an exploration standpoint, I think there's plenty of room to further develop Midgar, but in terms of narrative I don't see much call for it. You have the opening bombing mission to establish the setting and then you have a mission where things go awry in a way that flows into a chain of events leading to the escape from Midgar and pursuit of Sephiroth. You don't really need more than that to tell the story right.