OK but aside from espionage, which I'm not concerned about here, you only have to have met those civs in order to get an impression of what techs they know (by looking at their tech screen).
Of course I would deem it valuable to having at least met every other civ on the continent before tuning down exploration.
siggaboy,
I'm talking about an added bonus % of beakers. I forget the exact percent, but as someone who takes odd tech paths, I find myself backfilling techs quite often, and quite often I do this sometimes before anyone knows alphabet.
For example, last night I played the 34 civilization Earth map mod and I started as Ramses of Egypt. I started with Agriculture and The Wheel. Because I had Flood Plain Stone in my BFC, I researched Mysticism, Mining, then Masonry. I then grabbed Bronze Working, and snagged the four religion techs. By this time most everybody I had run into researched Hunting and Fishing, meaning both Hunting and Fishing were very cheap techs.
This part of the game is where this type of knowledge of this comes in handy. More about the game; a few civs already had Writing, so I instantly signed open boarders and traveled as far as I could, meeting as many civs as possible. A few turns later I had learned Hunting and Animal husbandry but I had not learned Pottery, Fishing, Sailing, Writing, Archery nor Horseback Riding, and nobody knew Alphabet yet. (I oracled Iron Working btw.) There was no tech trading happening at this point, but I still >knew< that many civs had Fishing.
Because Fishing has an arrow pointing to Pottery, that means I get another bonus to beakers while researching Pottery, so rather than hit Pottery right away (even though I really really needed it) I went out and learned Fishing first. Why? Because Fishing was VERY cheap. Most every other civ I'd met already knew Fishing, and it was already very cheap
because of how many other known civs already knew Fishing, and in a 34 civ game, you can run into a LOT of other civilizations in a few thousand years.
My next tech path will take me to Code of Laws, and I will stop 1 turn before founding the religion*, and point towards other things that will help me get Macemen quickly. This leaves a lot of techs to backfill (Monarchy, Feudalism, Calendar, Aesthetics, Drama, Music, etc.) Typically at this point the AI won't want to trade away these techs (especially Aesthetics) so the more civilizations that I know of that already know Aesthetics, the quicker I'll learn it.
It may only be a few beakers here or there, but a few beakers can be the difference between getting free great persons and wonders first, or losing them to a rival. When it comes to research, every beaker counts. Exploring is key to this.
This is just from a research-you-own-techs point of view of why it's good to explore. I didn't even go into tech trading, tech selling, resource selling, fog-busting, and the tactics of having regional and cultural landscape known prior to war. Personally, I always try to have 1 or 2 land units per continent and 2-3 sea units on continual auto-exploration (depending on map size), even after all the black is gone.
*I'll only stop 1 turn if I haven't founded the religion. I don't like to found religions on big maps because I don't want to use my production to spread my religion, rather I'll let the AI waste his or her production to spread their newly founded religion, and convert whomever they wants. This can cause tons of friction between many civilizations at once while I focus on spending my production on an army and taking over holy cities with swords.
