I don't think it'd kill the spirit of FF at all. I know people hate when these games are brought up, but when I think of a game without defined jobs, I think of FFs VII-VIII, or more specifically, the Materia and Junction systems associated. Other examples include FFX and FFXII. The characters had semi recognizable abilities hearkening back to classic installments, but the AP and License concepts allowed them to branch out.
Ideally, in this sort of play, you can't immediately /check a character and say "Oh, he's a RDM, he better be ready with my Refresh/Haste/Cure!" You can't instantly see a PLD and pass him up because you decide you want a NIN's shadows(which, as Seriha said, is something I hope S-E learned from), or instantly recognize a THF and pass him up for perceived lack of damage. It's possible, over time, to identify by armor, but if(like this game), multiple armor types look alike, that's not set in stone. Players will then have to discern others on a less broad basis.
When you don't immediately have a frame to put someone in(the frame being a specific class), it becomes less about what they are and more about what they can do.
Edited, Jun 2nd 2009 3:25pm by SunriderRagnarok
Level 75 Scruffy/37 Nerfherder
"The reasonable man adapts himself to the world, the unreasonable one persists in trying to adapt the world to himself. Therefore, all progress depends on the unreasonable man."
- George Bernard Shaw