[VanimaEdhel]

Refresh my memory: do they mention this expressly in the books or are you just taking this from the movie?

Well, ultimately, either way, shooting someone in the throat would probably be a very proficient way to kill him/her. Although it's a smaller area than the chest, an expert marksman such as Legolas could be pretty sure of making the target if he had a shot. When you are hit in the throat, you tend to die pretty quickly, if not instantaneously. Even if you do survive for a time, there is almost no chance of ultimate survival, especially in Middle Earth (no emergency rooms). I would surmise that, when possible, Legolas would choose such a target. Also, it is a place that tended to have gaps in armor (between the helmet and the chestplate), so you could find a place to aim that would hit skin rather than metal.