Bard Instrument boni

[Pyrion]

There are some songs where it's totally unclear for me how (or if) an instrument affects the outcome. Of course damage songs just do more damage, for speed and heal the effect is clear as well.

But what about the fear song (brass)? Or the enduring breath song (wind)?
The duration doesn't seem to change. My bard is 23 so i don't know about fear or charm yet. Is there any reason to use them with an instrument? I understand that the instrument skill is used in order to find out if the song fizzles (even if you sing it, which is really dumb), but is there any need to actually swap to the instrument for those songs?