"The Buckler Shield is a small hand-held shield. The Buckler offers less protection from ranged weapons but ensures the wielder stays agile."
For the Archer (Javelin), the buckler really opens up the class to a more aggressive role. The capacity to guard dramatically increases the class' usually low survivability - assuming the archer has learned to guard effectively. When the archer is out of javelins, you keep the shield for use with your secondary, keeping you in the fight for much longer than usual. Thus, it's a solid defense for an usually squishy class. A good complement to it is the Saber, for example, which can be a useful weapon due to its solid reach and speed for this type of situation when you run out of javelins.
For the Man-at-Arms, the Buckler's effectiveness as a shield in a traditional sense is debatable, as it does a poor job of covering one's body from arrows and consumes notable stamina on a successful guard when compared to the Heater Shield. However, the Buckler's guard is almost instantaneous, making it arguably the best shield for those who enjoy fast-paced melee combat (preferably paired with a fast-hitting weapon). As the decription implies, this weapon isn't designed for constant use (unlike the other shields), but more as an emergency measure when one is unable to dodge. Thus, manage your stamina wisely and the Buckler's value may start to shine.