The ability to have properly depicted civilians and crowds would add much more atmosphere and diversity to gameplay because now it's really hard (almost impossible) to create a city that doesn't remind you about ghost-towns like Pripyat and doesn't kill the fps at the same time.
For example patrol-type missions would be much more interesting.

To get the idea of what I mean just watch the video (or if you want to downvote it because "Arma isn't SimCity"):
http://www.youtube.com/watch?v=-D7KdmtdaV4