Lord Tirian
Erratic Poster
Part of the reason is that in Civ5, the WC finally gained some semblance of usefulness by integrating it with the spy system and the ability to trade for votes with other civs (doubly so with the community patch which expanded greatly on the WC). I agree that it was abusable, but that's something that can be fixed, especially if the underlying diplomacy system is improved.World congress was a heap of exploits and absolutely useless staff.
Furthermore, it actually gives Civs a non-military way to wield power. Trade embargoes or stifling science victories through economic vote manipulation adds variety to the endgame which all too often devolves into a "beat down AI" routine. It keeps the modern era fresh and unique.
However, I think it has to be done correctly and certainly deserves an expansion of its own - and I'd love to see the world congress not as a "thing" but a mechanic, meaning that alliances can form a congress and that there are multiple opportunities to do so with varying membership.