Originally posted by Moo
I don't find much use in keeping a computer around on your continent on Emperor level. In the Emperor games I have played, you can still do pretty everything like going for tech, military, wonders, and military at the same time without too many problems. I usually raze all the computer cities and build my own unless there is a useable wonder in the captured. I trade a few techs in the Ancient Age, and by the end of the Middle Ages, I am usually 1 or 2 techs ahead of the computers, all the while never setting the research rate over 60%. I never trade techs with the computer after the Ancient Age to always keep my military more advanced that theirs. I either buy them or trade luxuries. I save the massive D-Day style invasion until I get transports. By then I should be 4 to 5 techs ahead of the computer (should be only 1 major AI left at this point) and have been pumping out armies every 7 or 8 turns. I would land armies of Infantry on the computer as defense and use cavalry to take the cities. 3 Infantry armies should hold off anything the computer throws at you, and your cavalry will finish off the retreating fast units without suffering much damage. Since Infantry moves so slow in enemy territory, it's much better to load them onto the transport again and unload at the next enemy coastal city. Now, Deity is another story. I had to adapt to a very different playing style just to stay afloat like utilizing slave camps which I did not have to do on Emperor.
It makes sense that keeping another civ alive on your continent would not make sense on a smaller world, because everything is so cramped.
If you have no trouble keeping up in every way with the AI on emperor level, I am guessing that your are:
1. Playing on a tiny or small world.
2. Not playing with many opponents.
3. Playing a mod.
4. Not playing emperor level at all.
5. Really, really good.
It makes sense that keeping another civ alive on your continent would not make sense on a smaller world, because everything is so cramped.
On larger worlds, keeping a civ alive with 5 or 6 cities works wonders. I have often noticed ai civs that have nearly been wiped out, still manage to trade for the most up to date techs. Using this strategy, you can let your friend get them however this is accomplished (AI partnership?), and then get them from them with money or resources or luxuries.