Ever play Settlers of Cataan? By that logic you should never trade in that game either, except that trading is often the best way to win. As long as a trade is mutually beneficial, both parties will come closer to winning and thus have an advantage over the rest of the players in the game. Your theory on trading only works in a binary system. Of course you should never trade with a direct rival or a warmonger that has shown his true colors.
Trade partners are a good thing if you are on good diplomatic terms and are working towards the same goals. For instance, as Tarth I would always want ton trade with Pariden and Altar. This is because they will tend to be looking for an Alliance victory, which means we can win together. I would not be as likely to trade with Empires as anything I give them will be used against me as you have said. I would also want lots of trade with a faction that is currently very weak and of the same allegiance as me. I want them to be able to hire heroes and build armies in order to make war. Since I cannot attack them without losing my other allies, I must trade them resources to build up their nation. Trade is more complicated than you imagine.
The question the devs to need to ask is if their efforts in diplomacy will facilitate these extremely basic mechanisms. Anything less than what I describe would be a total diplomacy failure for the game. But we have no way of knowing much about the diplomacy intelligence until we see it in the beta. I am confident Fb has some smart mechanics in development.
I think this is spot on for what I feel trade should accomplish in a game, the thing I need from 8.6 trading is being able to trade over time (I give them 1 horse each turn, they give me 10 gildar each turn, (ofc. they should work against ppl breaking these treaties for instant gain).
Also I need the AI to realize what stuff is actually worth for both partners, I usually cant trade unless I get a ton of "Diplomatic Capital", which seems to be a freeway for free resources, and a free resource drain for a Computer opponent, since said "Diplomatic Capital" have no use in the game except grab all the enemies resources.
I would love to see the player trading expanded, but I don't like the idea to be able to trade with a marketplace, or npc store. Where I am not trading with another player that gain a benefit, but some wall of resources located on some far away island. I tend to find these kinds of trade's... Hard to translate, but they don't give me the eerie feeling of strategic victories, but rather another system to be "abused" to gain access to resources more easily, and would be another excuse to let the AI cheat abit more gaining free resources to hold up against a player using said marketplace.
Atm I like the idea that resources should be found physically in the world before accessible, doing so "Sparse" amount of resources will be a battle over the important strategic resources like iron and horses.
Sincerely
~ Kongdej