I usually play with tech trading ON.

After trying both my opinion is it's a whole element to the game you miss out on if you turn it off. There's a reason why there's whole super abilities and races with diplomacy as a specialty. It can be a powerful tool if used right.

About the races blending their techs together, I don't think thats accurate. The AI looks to be smarter in this game. At least at Challenging, I can usually trade for the standard techs I want if I have techs to trade back (this playing as human with uber diplo skills), but unless they are in deep trouble it is very tough to get a racial tech from them. Seems like they covet them as much as we covet ours.

Except you're not seeing / haven't figured out that the AI are giving each other techs hand over fist behind the scenes, leaving you out of it.