The Ranger is an interesting class, a sort of combination elemental weakness exploiter and really weak White Mage. While they get most of the useful White Magic spells early in the game (especially Cure and Cura), their proficiency at casting them, and their MP totals, leave something to be desired. You shouldn't rely on a Ranger as your sole healer.
However where Rangers shine is on offense. For the most part they will use arrows of the three major elements, and provided you switch to whichever is appropriate for a given fight, they can be effective attackers. They can wear decent armor while attacking from the back row, making them surprisingly survivable.