the reason bards have the healer LB is pretty clear: It's the only one of the four that fits both thematically and from an animation perspective.
melee DPS has them using the swords, which is already silly considering neither uses swords at any other time. The tank one and caster DPS ones don't fit thematically, which leaves the healer one.
In reality I would think the TRUE bard LB would be:
Level 1: A single powerful arrow on par with the melee DPS level 1.
Level 2: An arc slightly more powerful than the caster DPS level 2 but hitting a slightly smaller and more difficult to plot area.
Level 3: A ton of arrows slamming into the ground, basically the same as caster DPS level 3.