They don't let you sing 4 songs (without the harp) unless you have 4 dummy songs on already right? I don't play bard but that's how I was told it works.
This is 1 style of putting up 4 songs.
The other is putting up 2 strong songs(GHorn/Aeonic), then switch to harp apply 2 weak songs, then redo the 2 weak songs using Ghorn(or w/e you have better).
Both have merits depending on the fight style/conditions (death, jas, etc).
I mean, I think the original rationale was extra songs SHOULD be sung in extra song equipment (that's why it has duration so it can't be overwritten)
Except that doesn't work for obvious reasons and they weren't smart enough to code songs properly so we get imbalanced bard instead.
Then they just decide to let it go until someone brings it up and then they "fix it".
This really holds no weight. At the same time the harp was being released SE released a bunch of farmable +3 Instruments and gave us Emp gear with +1 to certain songs. for 30 to 40% bonus. Also they added the Empy Body (Aoidos' Hongreline +2) which gave a 10% Boost. So even if you didn't feel like farming the new superior instruments and wanted to keep rocking your +2 Ghorn you wouldn't have had a problem overwriting.
90 Emp harp was only a 25% duration boost.
99 is 30% So still would of allowed to be over written.
I believe the duration bonus was really added for AoE Sleeps since String aka Harps increase range.
Edit: They also added Aoidos' Matinee (BRD Aby Necklace) at the same time for another 10% duration bonus.
We already had 75 Mythic for 10% but they had already started the upgrade for even more duration at level 80 (20%), soon after 85 (30%) etc