His role as the sniper fits his personality and style. He's not great with confrontation, and would feel safer at a distance, much like a sniper would. Even early on in the series at times he has shown great courage to fight and push himself. Usopp might act like a coward, but when it comes to sniping he is to be feared as his aim is deadly. For his age Usopp can be considered a prodigy. For instance the dual with Daddy while the Straw Hat crew were in Louge Town, Usopp shows how excellent his aim was while he was under pressure, nervously doubting himself. His great sniper skills makes up for his lack of Physical strength, and his strategic skills are pretty impressive as he shows them during his fight with Luffy. You would have to be ignorantly blind to go around calling Usopp weak and useless, definitely the most underestimated character.
