Bard's AA steroid serves the exact same role as Sona's AA steroid: Increasing harassment potential. Bard is a harassment support: He doesn't really have the raw defense to protect an ally nor the hard CC to lock them down outside of his ult. His job is to win a war of attrition and wander around providing pressure to lanes: Because he only has the one damaging ability, he needs something to help him out with that goal outside of just cranking up the damage on that one ability. cough
cough
I hate the complaints about "Oh man, this champion has two passives, so OP, so complex!" It's the world's dumbest complaint. Do not judge a champion on its parts: Judge a champion on its whole. Does Bard, as a whole, seem OP? Does Bard, as a whole, seem overly complex? No. Not really. So it doesn't matter how many passives he has so long as those two lines aren't crossed. He could have a dozen passives, but as long as they were all simple, minor, and serve his character concept, it doesn't fucking matter. Complain about things that are actually a problem, like the ability to assassinate targets while building tanky, or the ability to escape from every fight, or the fact that an ability ostensibly about displacement and disruption has been reduced to a single target execution due to its stupid scaling.
Yes I am talking about single-passive-having Lee Sin right now.