FF14 doesn't have any real platforming elements to it, so I don't really see the need for a jumping ability... unless SE plans on adding said elements when they start overhauling the maps. That, or somehow implement it in a way that allows chocobos and other mounts to get to certain areas that would otherwise be inaccessible.

The only other thing I could think of would be if jumping would allow you to dodge Ifrit's Eruption/Radiant Plume attacks, but it'd really be difficult to time, especially with the current server lag in terms of syncing character placement and animation lock issues...