I think once you get to Deity the Spiritual trait is about as strong as the common favourites (philosophical, financial) if you take the time to use it fully. Its a whole lot more than just swapping CS for slavery to get some production in. There are a number of strong combos that come up when you unlock the civics (particularly strong with the 'mids of course): police state+slavery+vassalage+theocracy+state property makes for ridiculous military buildup. This is not something you would do without being able to switch in and out at will. representation+CS+mercantilism+pacifism makes for ridiculous GP generation and beakers, but you want to switch between this and slavery/OR to get infrastructure built. Even serfdom becomes useful, for example when you switch over improvements for state property, or unlock railroads, or even for calendar or just clearing jungle. That's not even considering the diplomatic bonuses.