I won't say it's a major issue, but it's a pretty big issue. No bard instrument mods is like a warrior without taunt or a cleric with no healing. Sure they have other things they can all do, but they're pretty crippled without those skills.
Bard songs work, sure. But anyone who has ever played a bard knows that certain things (especially drum based songs) don't work so hot without an instrument.
I can't code so I'm not going to tell you to fix it...but in my opinion, this is more important than the AA's everyone cries about all the time.