Phantom SP Build.

Only time its useful is when you already have HS and SE in your party. You can then put combat orders in 4th job slot or be selfish and use roll the dice/night lord 40 w.att buff. Unless the hs comes from another phantom and you dont have Adv bless ofc.

For a Marksman, it's great since Ultimate Strafe is 10 points, you go from 205% per hit to 215% per hit (about a 1.05x increase), but most jobs main attacks and buffs are 30 point skills, so you're going from say 260% to 266% on Hurricane for a Bow Master (about a 1.02x increase). There's a few other skills that can add damage, but not every job has those either.

Rage would be better for damage most of the time as a comparison. There's very few other 4th job party buffs to take over Combat Orders to begin with though. Speed Infusion is an okay alternative, but Phantoms themselves don't benefit from it, and it's only useful for classes that don't already reach the cap, which can be hard to accommodate for on the fly.