Static mate just made a Carn for our dual boxed BRD. Carn/Daru/Marsyas good enough, or should he make the Ghorn while we get the gally for Loughnashade?
Leaving S5 Prime away for a sec, I would say what's "mandatory" is by far Dau99 for 4 songs and then Marsyas for Honor March.
Ghorn is good and, most of all, it's relatively cheap and fast to obtain, so really no reason not to get it.
But "mandatory"?
You can get (total) Song+3 on other things like Linos or Miracle Cheer.
So really the difference with Ghorn is what? Missing Song+1.
Some examples:
Minuet5: 210att instead of 223
Minuet4: 190att instead of 201
Herc Etude: 23str instead of 24
Blade Madrigal: 108acc instead of 114
Carol: 144elemental meva instead of 152
I'm not addressing march because most of the time you only use Honor March (which is on Marsyas, so Ghorn can't be used).
If for whatever reason you're using double march (HM+VM) you're gonna reach magic haste cap regardless of being at Song+4 or Song+3.
Last but not least Ghorn provides additional macc for landing debuff songs, altough I wouldn't say they're extraordinarily valuable. Well threnodies on some V25 fights and on Aminon, for sure. For the rest it's an additional nice bonus but I wouldn't say it's the difference between failure or victory.
So, tl;dr, is Ghorn
mandatory?
Imho no, it's not. But it's so cheap there's really no reason not to get one.
Make sure you do have the HQ neck (song+3) though-