How about an avanced option for that, though? Say, you can choose a 5x5 grid that a soldier should watch closely, and if an alien enters the designated area, the soldier has a better chance of firing than if he'd just be on general reaction fire. Of course, he'd have no chance to react to aliens that appear outside that area. It may be a good tactical tool to have, for example to keep an eye on doors or corners you suspect enemies my come from.