I think the barrier to allowing other jobs to have instruments is that every instrument has a latent effect of "allow 2nd bard song effect". The clear intent of game designers was to make bard sub (and cor too?) a 1 and done buffer, maybe to encourage you to use a main support job player, since most parties wouldn't bother with a bard main if you can get 2 songs from a /brd... with another job's survivability/usefulness. Personally, I think the halved skill and the inherrent subjob gimping would probably kill any /brd for general play by 60 even with 2 songs, but that's just me.

Like I needed my bst/brd to have 12 (or more with elemental staves) more things to carry...