It's a cool aspect of diplo, and it adds much-needed depth to the game, with only one drawback: THE AI NEVER MAKES THEM.
Is there an XML where I can raise the AI's flavor for Defensive Pacts?
Yup, just finished a game as France, playing with Hukkak's civ pack.
Sharing a continent with Dutch, India, and Mughal.
Dutch at the north of the continent, went OCC/Cultural, ignored.
Ghandi and Mughal started spamming settlements, so I went down and settled their way, pissing them off.
Mughal declared war, I eliminated his army and started taking his cities. Overextended myself along the way, Ghandi declared on my backside, I pulled back my forces to my original cities and let him have my Mughal conquests, which were still so fresh they were in revolt/anarchy and not particularly useful anyway (population halved when I took them, then halved again when he took them).
Soon made peace with Ghandi, some time later he saw that Mughal was still cripped, and asked if I wanted in in devastating him. I agreed (his best cities were still ripe for the picking and closest to me), and SO, we shall GO to WAR against the Mughal, ripped him open, left him in tatters, with one useless tundra city. I declared peace after XP farming him until Ghandi was closing in on the last city, as I didn't want the warmonger penalty, and oddly enough Ghandi did as well, leaving him his last useless tundra city to shake his fist in rage for the rest of the game.
And so effectively, Ghandi and I were sharing the continent, about 20 cities each. I proposed a DoF, which he agreed to, and then HE proposed a defensive pact, which I agreed to.
We then kept a DoF and Defensive pact for the last 300+ years of the game, with someone I HAD BEEN TO WAR WITH, until I launched my spaceship. Whenever he asked for a declaration, I was in - who cares, whoever we were "declaring" against was across the ocean somewhere and no threat. I did a couple of proxy unit donations to my CS allies over there to keep them alive and make it look like I was helping, but that's about it.