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.