Monasteries are required to build missionaries unless Org Rel is one of your civics. Yet once you discover Sci Meth, monasteries become obsolete, so unless your religious civic is Org Rel, you can't build missionaries in the late game. I know the upkeep for Org Rel is high, but the benefits are enormous. I *never, ever* build monasteries; in the early game I tend to beeline to Monotheism, adopt Org Rel, hurry a few missionaries, & send them to my rivals before they convert to a religion other than my state rel. Then I spread my rel to my own cities so I can build temples, hire priests & generate Great Prophets. I tend to play at Settler so I can found all 7 rels, & last game I played, I managed to spread all 7 to almost all my cities shortly before (or maybe shortly after) I built the UN. So by the time I passed the res to make Free Rel global, I no longer needed missionaries; the diversity would provide +7 happiness per city once Free Rel took effect. So all in all, the only religious civic I find useful (until late game) is Org Rel.