dh_epic said:
Every civic has a purpose. Well, except Environmentalism and Nationalism. Haven't found uses for those yet. And slavery is a little iffy, too.
Slavery saved me - at least for the early going - when I had a Flood Plain start surrounded by little more then desert and 2 or 3 grassland. This is incredibly specific, but my population was running rampant once farmed and the only way I could build due to the VERY slow production (30 turn barracks, for example) was

my buildings into existence. Between the health & unhappiness, I was forced into this.
I also tend to use it if I have fast growing cities with little to no luxuries to - ahem - make use of the unhappy people

. That being said, in most cases I won't switch for JUST this civic (unless Spiritual, of course) but once I can do Hereditary Rule, Organized Religion & Slavery or another multi-combo change.
Nationhood - Another example - Warmongering and overexpanded, had an 'ally' declare on me on my weak flank. A quick switch and some 'conscripting' and my 'back end' was protected within 2 turns. Made Peace at the vanguard and redirected my military to the new threat.
I've also used it 'Sirian-style' for my once-isolated-civilization-turned-tasty-target and get 2 defenders quickly in each border town. It was nice to have so I could get back on track with my current agenda.
**Sorry to the OP for threadjack**
Theocracy is great for multi-player war & negating religious spying as I don't find the AI uses the religious spying to the fullest. Pacifism is perfect if you have GOOD (re: not heavy) defense set-up and are looking to seriously up the Great People points..