What is the tactical reason for doing this? would this not risk the vassalled AIs trading off crucial techs to independent AIs?
Since you can control what they research, I usually I want my vassal to be "caught up" enough to be able to research a tech (like biology for instance) while I go for stuff like rifling. I don't usually give them everything but sometimes they're so far behind it doesn't matter. I give them enough to tech what I need, since usually my tech rate is far faster than theirs since I'll usually have 20-30 cities to their 3-4.
With that I can use the vassal to backfill my techs and save me the 2-4 turns it would take me to get it. Back to the example above, my vassal can research Biology, while my other one can go for railroad for instance. They'll take about 10-20 turns to do that since they're slower (Used those techs as an example since by time we get to Scientific Method I'll usually have a vassal or two).
That'll keep'em nice and busy and with that, while they take their sweet time to tech Bio and Rail, I'll get communism for the free spy and state property. Depending on how fast my tech rate is I'll even go Physics and start another gold age with my free scientist and spy (my first one is usually with the free GA from music, unless I build Taj Mahal)
I'll plow through Electricity, assembly line, etc. Eventually I trade my vassals something like Electricity for Bio and same with Railroad (sometimes I have to give them multiple techs but doesn't matter) then my vassal can go Fascism to backfill while I keep teching ahead.
Also like Gumbolt said, it helps when the vassal has the same military techs as me so when I go to war to conquer the rest of my continent they can offer a wee bit of help.
Also the diplomat bonus is a plus
In the same vein, why gift the capitals back to AIs?
Because usually I'm at the point where I'm pretty much done with the game and it's won. I have so many cities and I'm not too keen on dealing with waiting another 7 turns for the revolt to end, then the culture wash, then all the "motherland" stuff. In this case, I just did it out of good sport since, if I recall, Egypt was friendly almost the entire game and I had to backstab him to win the game so I just gave it to him because I felt bad for him lol
Usually with my first few vassals (not counting AI's I killed early in game) I
keep the capitals since they're on good land (as Gumbolt said) but after a certain point the game is usually about to be won so it doesn't matter to me.
On the other hand, it does have its strategic uses for me. Besides skipping the 8-10 turn revolt, it helps my already weakened vassal's tech rate for the reasons I mentioned above. Don't want'em taking 30 turns just to get Biology since by that point I might as well tech it myself!
Especially when I take cities on another continent for domination...like I just want to win. I don't want to deal with 30+ cities and then
another continent with a starving city that only has 3 workable tiles because it's culture-washed (and the game is pretty much won anyways at that point so it's pointless haha)
Plus another city to make units for our wars and the diplomacy bonus helps as usual. I've spoiled my vassals the last couple games and it has worked to my advantage but that's just my strategy at this stage. For space race
then I'd go for cities rather than land because I hate dealing with revolts and all this "yearn motherland" nonsense lol.
I haven't done a space race in a while since they're so long and drawn out it feels like
forever. Don't get me wrong- I'm one for playing SimCity but even then...micromanaging 30-40 cities can be a pain for a busy man like me. Like 10-20 minutes each turn...then there's my 50 workers to deal with... I don't even wanna imagine a huge map with like 100 cities...on
MARATHON. haha nooooo. Capturing cities is just fun