I'm guessing they start out level 1 and gain exp like a party member - that is, if you kill a too weak, the NPC gets no exp regardless, meaning you having to do stuff from level onE and up..which will probably be really tedius...because they'll probably fight like a person with auto-attack and nothing else, via Escort-For-Hire NPCs. Might be a good idea, but I still bet it's gonna be incredibly tough for it to become remotely worth it =\
But a chocobo NPC partner would be neat...or one of those Goblin vendor-guys =P