I'm pretty sure there's one character in the main story quest line that switches classes in a very obvious manner at one point in the story, but won't say much more than that in case it could be considered a spoiler. I tend to theorize that they are just set to main certain things, and that a number of them main healer due to necessity as the player character won't always come to single-player quest fights as a healer class. That it probably follows with the design of the player-character class/job system -that the developers can set npc's to whatever class they need them to be for whatever length of time, but that it's probably easier to just include an npc that already is the needed class in those scenes or fights.