Yea, they need some kind of limitation. Whether that means a seesaw effect, or some kind of ability to set a weapon as your main, and not being able to do this in the field. That 2nd way would allow you to master everything, but you could't be a master of everything all the time. You'd have to choose something before hand, and then all other jobs you had would work something more like a subjob system ver. 2.0. The difference being that you could switch to any of your backup jobs at any time. Just not as powerful as your current main. If something kind of limitation isn't placed on it, then I am afraid it will be as another person said: everyone would be "required" by community standards to have everything mastered. I like a certain degree of specialization. I don't want to get in a party and have everyone tell me All jobs mastered or GTFO. The key here is the freedom to be a jack of all trades, but a master of none. Not a master of all trades.
In addition to some form of elitism forming around being a "Jobs Master", this would also lead to a lack of (for lack of a better word) team spirit. I will quote a player from a game I played, that had abilities that made it where everyone was basically their own army: "Heal yourself noob!". And this was said justifiably so, because if you have the potential to take care of yourself, then you should. So I see where someone with this view would be coming from. All I am saying is I dislike this idea, and do not want it in FFXIV. I want a strong and helping community.
Lets just say everyone can become a master of all jobs. And lets suppose that as soloable as FFXIV is, that some quests, mobs, missions, whatever are hard enough to require a group of "Job Masters". So they will work with eachother to achieve their goals, but not as a team. Because they are each so uber and badass, they do not need eachother's help. So while they are working with eachother, they are not working for eachother. Each is doing his or her own actions. If its magic resistant, everyone will melee. If its melee resistant, everyone will nuke. If everyone takes massive damage, everyone heals themselves. If someone dies, your on your own. I am not saying this is positively the way it is going to be, but FFXI made people dependent on eachother. FFXIV sounds like it is trying to make people so self reliant, that it will be like a lot of other MMORPGs where there is no sense of community or team. And why should they be helpful or have any sort of want to act in favor of the team? They can solo everything, and frankly don't need you. This is all speculation though, we don't know the dynamics of the battle system yet, and they can still put in those limitations.
Also, Cannoneer....... just sayin :)
I also want double eyepatches, to increase my pirate bravado.