The US loves to sell weaponry, we all know this, but the US doesn't love to sell weaponry to what we perceive as our rivals. Hence my reference to China. But why don't we? Because they know, that if you sell valuable equipment and expertise to a rival, they will develop their own independent capacity to use such technology and enhance upon it much more rapidly than otherwise.
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I don't really see how this is relevant to the discussion tbh. It's exactly the same whether you use a license model or a direct trade model in this instance - if you don't want to trade them to others, then just don't sell them the license in exactly the same way you presently don't sell them the tech.
Also, the reason the US doesn't sell arms to China is due to the post-Tienanmen Square arms embargo. From 1980-1989, both Europe and the USA sold vast amounts of high-tech weaponry to China, and would probably still be doing so (like Russia quite gleefully does, despite seeing China as their biggest security threat - and in fact China still acquires lots of US weapons systems via Israeli licensing and on-sales).
In every instance you mentioned, the result has been the recipient country understanding the technology and being able to replicate it on their own within a short time frame (lets just call it 5-10 years, yes, I consider that short). Considering the advanced modeling computing technologies that any Civ in GC3 possess, not to mention the collasol scale of resources even a 2-3 planet empire enjoys (compared to our current nations), there is every reason to believe that 5-10 years would be truncated/accelerated significantly.
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Alright then. In the 1500s, Europeans sold guns to the Native Americans. Three hundred and fifty years later, not one Native American nation had developed an arms industry capable of producing a single original rifle, even though they used them in large numbers. If you want to design a decent gun, you need to have a good understanding of several different sciences - metallurgy, ballistics, chemistry, engineering, mechanisms. And that's really just a sealed tube with gun powder in the end - consider how many different sciences you need to know to design a microprocessor, or an aeroplane. Are you putting all of that stuff into the box?
Also, your argument kinda supports my point that real-world tech transfers occur through a system much more like licensing and much less like diplomatic tech trades.
Also, consider the expense of shipping actual components in the GC3 universe... and their communication capacity seems to be rather nice, so why wouldn't you sell a transmission filled with technical information? Shipping the components and such would come with increased overhead, and if you believe they would end up understanding the technology in just a year anyways.. why not save the resources and trouble? You don't sell tech to your enemies, so you are not THAT afraid of how they are going to use it... and if you are... don't sell it lol.
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Well, I'm already sending out massive freighters full of something or other, so why not arms? Why would I be willing to ship over a thousand tons of clothing, but when it comes to my latest top-secret weaponry plans I'll just give them away instead?
Or if you want to go the transmission-only approach, why not send him limited use 3D printer files? Just add a destructive read subroutine; this can already be done with modern equipment. Send him 50 Read Once blueprints a week until I decide to stop doing so. And why are you assuming I'm only interested in selling licenses to my enemies? I'd sooner sell licenses to everyone. I have permanent interests but no permanent allies 
Besides, I may be 3 or 4 tiers of tech ahead of some AIs who I wish to sell licenses to. This is selling rifles to the natives again. Quite frankly, there's nothing to say you won't get Doom Rays before the other guy has picked up Matter Disruption; it's likely to rely on branches of science the recipient hasn't even heard of yet. Kinda like an inverse Outside Context Problem; the weapons are so powerful that the recipient can't even begin to understand them. He just knows that they come out of his primitive 3D printing device and work.
I'm guessing we are not going to agree here. Which is cool. as a game feature, it isn't bad, but I just don't see it as a must have priority when we have SO much more stuff that needs help.
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Well, it's got potential to help with diplomacy, which is currently massively crap and is exactly one of those things that does need sorting ASAP. If diplo wasn't screwed up already, then I wouldn't be suggesting changing how it works to improve it. Licensing as a tech trade system has been tried in other games, and worked well; it prevented the worst excesses of player tech trade abuse while still permitting the player to pick up advanced tech from other players.