It's somewhat situational, and I'm using Raging Barbarians, but: I tried going for religion and heathen conversion early on. I thought it would give my missionaries the chance to convert barbarians by using Spread Religions. I was blown away when I noticed just bringing the missionary next to the barbarian unit was enough: One missionary can do infinite conversions! My location is relatively heavy on islands, but Communitas maps tend to be. I have about 10 barbarian ships in the seas around me, based from on camps on islands around me. It looks like I'm about to get a Classical age invasion navy/army of ~20 units with just a missionary or two! And I can keep farming camps for units! Really it seems to me that converting heathens should spend Spread Religion uses. It would still be a good option, converting 450 faith (a missionary) to 2-5 units (if missionary converted all adjacent barbs when spread was used, and each missionary had 2 spreads). Seems on par with buying units with faith.