I guess I could see this being worthwhile particularly in the pre-industrial era (when you can't buy GS anyway), but thing is I rarely buy missionaries.
Well, this is the thing about the religion system - you can tailor the religion to the strategies you do use. In my current game I just got 110 science in 1200 AD for spreading the word to the 10-pop Iroquois holy city (and it doesn't seem to have harmed relations, since I'm not converting their city to my majority religion), which isn't bad for a 200 faith investment on a missionary who can potentially do it again.
I think it's a backwards way of looking at things - "I don't need this because I don't build missionaries". Rather you can look at it and think "This is an ability that allows me to gain science if I make missionaries, so I'll make missionaries". It's the same thing I've noticed with people dismissing the Honor policy that gives you happiness from defensive buildings ("Well, I never build them"). You don't take the policy because you build lots of walls, you build lots of walls precisely to exploit a policy that gives you access to a full chain of no-maintenance happiness buildings that can net you up to 4 happiness per city without any gold cost over the course of a game.
Getting to your religion early + spreading it strategically with one or two missionaries is usually enough to dominate my continent, combined with Religious Texts, and I'm then buying up cathedrals & pagodas with my remaining faith. If I max those out before I hit Industrial, I'll let it go for another great prophet, which I can send out with a caravel escort to convert overseas CSs.
A missionary spreads faith two or three times and you can get multiples - you can only have one cathedral per city (I have taken that belief as well - playing as Korea, an extra - and maintenance-free - specialist slot in each of my cities = +2 free science per city, in addition to my usual tech bonuses, Messenger of the Gods pantheon and interfaith dialogue-spreading missionaries. And somehow I'm still 5th in literacy...).
I will say that one of the things I like most about the religion system in G&K is that there isn't a clear "best" strategy, at least not yet, and it really does behoove you to change up your go-to strategy depending on the mechanics of that particular game. That said, for a primarily peaceful strategy I'm finding it hard to beat Tithe, Cathedrals, Pagodas, Religious Texts (plus whatever pantheon suits your start)
I find I'm generally choosing different ones every time - not consciously to try them out together, but just because of my particular situation in each game. In this one, for instance, I got religion very late so Interfaith Dialogue should have been ideal (unfortunately all the other religions seem to have been founded by civs far away except for the Iroquois one, and theirs was founded later than mine). Though with Cathedrals and Guruship, I have tailored my religion somewhat to playing as specialist-based Korea (and as the Maya I would usually take GP-related religious bonuses), and I've also expanded much more quickly than I usually do partly in order to maximise my science bonus from trade routes (I have 5 native cities plus Attila's Court by 1200; only Attila's Court still needs connecting by roads since it's my newest acquisition).