One question you should always be asking is what are the downsides to failing an Arcane Art roll? Sure, failure might be rare given a high CHA, but to what does it expose you? Given that bards are out there, I would presume that every NPC is aware of their general abilities (just like for a Wizard or a Paladin). Fail a roll and suffer a hard move? Maybe you strummed a grace note and opened your mouth, only to have someone chuck a dagger into it because they know you're trouble.
Even if you succeed, what does it mean to be standing there singing/playing/orating in the middle of a fight? Speaking as the hypothetical NPC fighting the guy you've just healed and/or buffed, if I smack him and I see that you sing a little pep-talk and he gets back in the fray with renewed vigor, my next attack is going to be targeted at you, not at him.
As for adding to the fiction, you might want to consider ways to make your Arcane Art have more tangible fictional effects. Rather than just a largely mundane dude playing a rousing, inspirational tune you are actually a Spellsinger, and you are effectively casting magical spells with a specific verbal, somatic, and/or material component (i.e. your lyrics, your playing, and your lute). It is called Arcane Art after all. Like any other magic, maybe it has visible, fictional effects. Maybe the reason someone gets a buff to their damage is because your music is making them dance through their opponent's attacks to land a hit on a weak spot. Maybe the reason they're healing is that your music is a direct appeal to their patron deity. Or yours. Depending on the patron deity in question, that might have all sorts of ramifications.
But you raise a valid point in that the Bard is quite possibly one of the most poorly-thought-out character classes present in the (A)D&D X.Y canon. I can't even blame the DW writers for this because they have tried to faithfully capture the functional mechanics of a character class that is at its core pretty lame. The old "sing to give your buddies +1" thing just lacks...anything really. But mostly panache.
That said, I have played a Bard in D&D and it was one of the most fun characters I have ever played. But I think I can count on one hand the number of times in the entire campaign that I used the buff. The ability that I used most frequently was charming a crowd (such that my partner the thief could more easily pick pockets in whatever inn or tavern we were working). In combat, I was usually trying to be inconspicuous and sneak past the bad-guys in order to get first pick of whatever loot we were after, generally using my party-mates as a distraction. But what made the character the most fun was almost completely RP, and that was that I was a total wastrel. I had a well-established "weakness for the ladies" that got us into all sorts of fun trouble. And we'd roll into town after some haul and all of the other characters would be like, "I buy magic armor," or "I buy Boots of Speed," or "I research a disintegrate spell." My response was usually more along the lines of "I spend [rolls a random die] 800 gold pieces on ale and whores! Now keep in mind that I only have 600, but I spend 800. I figure after the first 2 or 3 hundred, someone will extend me a line of credit. Boy are they gonna be surprised in the morning. Also, some time before sun-up I roust you guys out of wherever you are and whisper, 'Ssshhh. We need to be going. Like now.'"
Because my character class abilities were generally so lame and ridiculous I took it upon myself to become less of a mechanical advantage and more of a plot device, and in doing so had an absolute blast.