Ghpstage
Deity
You can gift units in an AIs territory to that AI, it isn't limited to permanent alliances or vassals/masters or anything. The advantage is that the AI gets a 50% discount on upgrades.Ah, thanks. Interesting...
I'm not sure what you meant about "send them my old units to upgrade", though.
You mean, I move my old units to their culture area, then I can upgrade my units if, for instance, they already have an upgrade I don't have? E.g.: I haven't researched all requisite techs for Cavalry, but they have, so I can send my Knights there and upgrade them to Cavalries?
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The other problems are,
That it screws up your diplomacy as AIs dealing with you will use the average of what they think of you and what they think of your ally rounded down (just like they do when you have vassals) and the GUI doesn't tell you this. It can lead to a lot of ridiculous situations, such as worst enemies at 'friendly', but at least its more understandable with permanent alliances than vassals.
The AI, having access to all your techs will happily give them away to other AIs....
Considering the late game tech requirements, and need to share a war for so long I don't think i've ever seen a situation where a PA would actually be worth it.