There is all kinds of magic you can do with SPI.
Whipping units (in your capital while in burocracy for +50% bonus hammers too), then leaving the units in the queue and releasing them with vassalage is just one obvious example.
Or whip some needed infrastructure (say barracks) with org religion, then throw the overflow through theocracy boosted units.
The next 5 turns, you spend in serfdom clearing jungle while cities fill up their queues with a few hammers into a xbow, a mace, a pike, a treb and a catapult. ^_^
In some ways, it's better to describe SPI as a trait that gives you more civics than just the ability to swap between them.
Whipping units (in your capital while in burocracy for +50% bonus hammers too), then leaving the units in the queue and releasing them with vassalage is just one obvious example.
Or whip some needed infrastructure (say barracks) with org religion, then throw the overflow through theocracy boosted units.
The next 5 turns, you spend in serfdom clearing jungle while cities fill up their queues with a few hammers into a xbow, a mace, a pike, a treb and a catapult. ^_^
In some ways, it's better to describe SPI as a trait that gives you more civics than just the ability to swap between them.

, as I really like his unique stuff. But he is extremely behind long time and catches up only later. Toku has ok starting techs and a solid UU that neutralizes the Longbows first strikes (they survive city attacks more often, you can attack w/o siege, if there is a castle, you need siege though) and if you play overall aggressively, he is not bad, you can keep your research on a high level due to conquest gold (even razing one or two bad cities becomes an option).
..although starting techs are not ideal.