Morcar_olmig
Warlord
I guess it's a good option for OCC games?
If you have big holy city (it's almost not affected by missionaries, thus can be hit many many times) of another religion nearby then it might be worth. For example for the city with 20 foreign followers you would get 10x20 = 200 science for one religion spreading - so one missionary would worth 400 science or 600 science with borobudur. Great prophets are more useful as they have 4 religion spreading possibilities, but their price goes up and they eliminate foreign religion - what might be ok only for non-holy cities. On the other hand when you enter Industrialism you no longer want to spend faith on missionaries (exclusion is the end game when you don't have enough faith to buy any GP).
Thus interfaith dialog is useful mainly until Indu and with strong religious leader nearby.