AT Weapon: Helps you recognize and use anti-tank weapons like LAW rockets. A handy skill to have should you encounter something far tougher than your Ranger instructors ever told you about.
AT Weapon however suffers from two major drawbacks: lack of ammunition, and its ability to only hit one target at a time (granted it hits them hard). Rockets can also be a bit unwieldy and often miss its intended target if the range and skill level are not right. Such an instance can prove expensive, and even fatal at the wrong time, so it is best not to rely solely on this skill to eliminate most enemies.
Demolitions can sometimes prove more effective, especially when dealing with groups of enemies.