Where is the bone horn rule from? Sounds pretty powerful for something you can get practically for free.
The bardic ability may only get to +2 at 8th, but you only get your first couple of 3rd level spells at 7th level. In my opinion, it's worth saving a known spell to trade for the bonuses to saves. There are *so* many good 3rd level spells for a bard, I just can't justify using up one of my 2 or 3 3rd level spell slots for a spell that is almost completely duplicated by a bardic music ability. Haste, displacement, glibness, slow, crushing dispair, dispel magic...
And that's not even taking into account the bardic music boosters, like words of creation or this natural horn thing. If you count those, the music is actually better (unless you really need the save bonuses).
-The Souljourner