Ok, look, I can really understand why people have the view that Tech Trading isn't good. This is because it's never really been handled correctly in any game I don't think. If you're going to trade tech you need to put a little "realistic thinking" into it. In real life if two countries were going to trade a tech, say a powerful weapon, the country that wants it is going to be paying through the nose. The country that's giving it up obviously wouldn't give up their best weapon unless they had a bigger better weapon in the works or already done.
I'm what you might call a "Completionist" ruler when I play games like this. I want my nation to know EVERY form of research there is. I want them to know how to make Every weapon, Every Defense, and have the best of the best of everything by the time I finish the game. That's driving force number one behind wanting tech trading, and my own personal view. I do have a more logical view and argument on it which follows.
If your civilization didn't know how to make a sword and you managed to kill a few people who had swords, your people would pick them up and take them. They'd study them. They'd keep getting more and more as they killed for them. Eventually they'd figure out how to make them on their own when everyone else around them had them and they didn't. It's only logical as time progresses that civilizations will emulate one another until they all have the best technology that can give their citizens the best life. This happens in the Real World too (no not the MTV show).
When one country started using planes to fight wars other countries realized the advantage they had and started doing the same thing (WW1). When one country started using tanks to cross the battlefields to get their men into the enemies trenches suddenly every country started studying how to make and use tanks (also WW1). Technology also spreads through espionage or through assimilation. When you conquer a city that belongs to a rival faction, unless they burned the city to the ground and left it to your advancing forces, the technology would still be there and the conquerers could study it and learn from it until they knew how to use it themselves.
I really think Tech Trading also brings a LOT to the table in Diplomacy, which is highly underused and underrated in game as it stands now. The thing to do to keep it from turning into GalCiv 2 Tech Trading is to make the AI Highly Value their techs and only trade techs that are weaker than their best techs. Also have them Not trade their very best techs No Matter What The Offer Is. No country, and I repeat NO COUNTRY, would trade away a Iron Sword unless they knew how to make Steel Swords. Once they knew how to make Steel, they would be more likely to trade away Iron, because they have better and the advantages of sharing the Iron Sword could far outweigh keeping that knowledge secret.
One of my favorite strategies is to give techs to a weaker country to get them to fight a war against a stronger country that I don't want to confront directly. Usually you do this through minor civilizations. Give them tech that's slightly better then their much bigger enemy, then have them whittle away at that other country until they are weak enough that I know I'll win in a war. Then I go in when both those countries are at their weakest and finish them both. Without the ability to trade (or even give away) technology, that strategy isn't open to me At All.....and I don't like that one bit and find it Very Limiting in a game that's supposed to be about Grand Strategy and World Domination. Sometimes a direct war isn't always the best way to win, but bribing a weak nation to attack a strong nation without some way to even the playing field just ends up with a even stronger strong nation that I wanted to take down to begin with.
I would ask those that are against tech trading to at least think about what I'm saying here and instead of just saying "no, I don't like it because it wasn't done right in the past" to try to think of a Fun Way it "could" be handled that You Would Like. I honestly think it could benefit us all and the game greatly.