That's not where I was going, per se; but in case people are thinking that, I guess it's worth clarifying that I think this sort of thing would work as a limited job because then leveling other jobs to use their actions and traits on Freelancer would be optional. That's very different from, say, 2.0~3.56 where playing multiple jobs was, practically speaking, required because of actions like Quelling Strikes and Provoke, or to unlock jobs (the worst offender probably being PLD because it required levels in CNJ just to unlock it, even if you weren't ever going to use any of the CNJ spells).
I don't want people running around with incomplete actions sets if I'm going to be matched with them in DF--this is already at least a bit of a problem now, as I've seen BLMs in Lv70+ dungeons who don't even have Freeze and Blizzard III on at least three occasions. So a Freelancing system would necessarily have to be Limited.