I still think the simplest way to make DC work is to give it concrete (practical) value for both humans and AIs. Right now it's a gimmick that only has value because the AI is forced to think it has value - and don't get me wrong, I like this gimmick (the idea of it, anyway, if not the implementation); it's already more elaborate than many games' diplomacy systems, but it could be better. I feel like I'm exploiting the AI if I ever trade him DC, because the AI is giving away real practical resources for something that has absolutely no use, the AI just happens to be suffering under the delusion that it's good for something.
So.. why not make it good for something? The thread Mandelik links to had many great ideas for this, just to recap:
-Ability to "buy" champions with DC instead of gold.
-Other neutral units that can only be influenced with DC - i.e. go to a Troll camp or a Dragon nest and buy units with DC. Flavorwise, think of it as using your influence to persuade some of them to join you.
-Unit enhancements that cost DC - say an assassin's kit with a DC cost you can add to custom units. Flavorwise, think of it as using your influence to get special training for your units.
-Abilities that cost DC: say your sovereign can make an inspiring speech to your people, this propaganda loses you influence with other nations (costs DC) but raises your own nation's population growth or somesuch.
-And I'll just add the excellent suggestions in this thread, to be able to persuade other nations to enter trade agreements or peace treaties (or even to marry their children to yours, if dynasties ever get any real purpose) by spending DC.
Suffice to say, there are many ways you could add practical value to DC, and all of a sudden it'd not only have a practical use for that poor AI sap you traded it to for real resources, but even humans would have a use for DC. Can you imagine the AI using diplomacy to manipulate humans for a change? This could happen if DC had a concrete value that even humans could make use of.