I'm not sure where Spiritual shines most. It certainly has its applications for Diplomatic and Cultural victories... but I feel I get the biggest mileage out of it in military games.
On high levels, the diplomatic benefits are great since a war ally is incredibly useful... and it's more solid economically than usual because onewants to switch between peace and war civics repeatedly.
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Off a tangent: That's one of the reasons I dislike the military traits so much - I wouldn't even prefer them to SPI/ORG in a game where conquest and domination were the only victory conditions.
The traits themselves are not enough to let me go to town without a military tech advantage, and they aren't packaged with UUs in a way that allows them to shine.
Aggressive Praetorians or Protective Skirmishers would be sufficiently awesome, but we don't have that without unrestricted leaders.
Churchill's Redcoats come so late that we may prefer one of the queens for a Renaissance bloodbath - they simply get there faster. Cho-Ko-Nu are fine units and benefit from Protective... but good enough to be spectacular? Of the Chinese leaders I prefer Qin because CKN make an Oracle/Metal Casting slingshot more attractive... but he doesn't measure up well to Augustus. Praetorians don't require one to jump through hoops to get them early and imo remain more consistently useful than Cho-Ko-Nu, even without a combat trait.
Samurai are the only case of a powerful UU being boosted by Aggressive... but there are a lot of problems there as well: Stricter resource requirements, weakest economy of all leaders, double-boosted Musketmen being more than useful enough during part of their lifespan and, for the AI, a crippling unwillingness to trade.
I wonder whether part this is deliberate in order to make it less likely that a human player will have to face an unstoppable military juggernaut.
I'd not be so quick to dismiss a promo advantage, since it can give a very respectable odds-lead at parity:
1. AGG melee, as well as AGG or PRO gunpowder, gets access to formation instantly. When knights or cavalry threaten to flank your siege, this is not insignificant at all.
2. The combat promo on maces in addition to CR II gives them a considerable boost vs longbows. CR II strips a longbow defense 45%, but with just 33% defense bonus left over after the longbow is still higher odds! If the mace has 8.8 str though (combat I in addition to CR II), the longbow needs an additional 13% (46%) defenses to hit break even. The first strike won't overcome that variance as easily, and without fortification you actually beat the 45% from CG III, not that such actually happens :/. This can also give the maces a nice lift after even minimal collateral, and combat I CR III is quite scary and if you're running max XP civics not far off.
3. Combat II knights will tend to fall very easily now
4. You are nigh-untouchable in the field at tech parity
The synergy for military traits is to pump their XP even more, so that they can easily reach 10 xp. Combat IV or III + a counter promotion, CR III, or drill IV are all setups that give a stock unit a considerable lift vs same-tech opposition. Combat II pinch rifles for example fight at 16.8 str and strip 25% defense from the only defenders you'd care about at parity. Only a rifle with CG II AND fortification bonuses or better will have winning odds vs you, and only at full health.
Having amphibious at 5 xp is a cute trick too.
IMO the AI bonuses help make this harder to use, but vassalage/theo troops are still good enough to abuse the AI below deity with AGG or PRO (gunpowder +). Cho Ko Nus are an interesting option if AI can be blocked from knights/cuirassers, because they are generally devastating vs non-mounted with minimal siege help. They're no prat, but still usable to good results.
Would I take them over better traits? No. However, they do have material benefit in war games and allow easier execution of a different style of war, which is fun as a changeup.