I see the major advantage bring role versatility. A Lancer could be made into a tank for example. Though this would really only work if you had a static make up for it. If you queue as a Lancer, your party will be very confused and annoyed at it.

Classes offer versatility while Jobs offer specialization. Most people just look at the stat boosts that are lost, but they ignore the utility. The armory system is still too in it's infancy for it to be tried though. Perhaps in the future we will be seeing a healthy mix of classes and jobs running end game content.