The reason the prophet is necessary is because:
a) that's how religions are actually founded in RL.
b) I love and wish to retain Choose Religions and Prophets allow for this where missionaries would not.
c) As noted, the 100% success rate for spreading for the prophet is built into the code where it would not be for missionaries, making it quite possible a religion may vanish forever in the failure of its first and only attempt to spread.
d) I like exploitability. But then, if anyone thinks that doing anything BUT founding a religion or at least building one of your religion's shrines, until all religions are founded (except Mormonism) is a better way to use a prophet, I disagree wholeheartedly. I personally believe it shows a lack of respect for the power a religion brings to allow an opponent to grab it rather than take it for yourself. Few benefits in civ could outweigh those of pretty much any of our religions. The culture only shrines are a bit weak, true, but I have some plans that will make them extremely desirable, perhaps moreso than gold.
Also, there are still a few ai fixes to come here to get most ai to play more like a person, founding the religion a bit faster.
And since we keep questioning the decision to make religions found by prophets here, I'll have to take a step back to this and develop out the next phase of the plan here lest I get too frustrated, then if you still want missionaries for this, I'll build you another option.