A roof top is a great place to huide & guide because you can position yourself in such a way that it is fairly easy for you to fire down on your foes but it is a little more difficult to manuever a missile up into you location. Of course when playing regulars this doens't really apply. A couple drawbacks to rooftop hiding. (1.) missiles coming from a lofty area are pretty obvious and is a sure give away as to your location and (2.) there usually is no place to hide if someone does get one up there.