PLease don't forget that Classes > Jobs at the same time Jobs > Classes.

Its all circumstantial if its just me and two friends in a party doing leves or something along those lines then I am going to probably suggest we all be classes since we will have the most freedom to equip ANY NON-EXCLUSIVE ability/spell/weapon skill that could potentially make us stronger and able to do said content.

Vs

If I am in a full party I will probably suggest we all be our Jobs since they are more specialized and we have the man power needed to not have to spread out our abilities.

Because don't forget a Dragoon (job) will probably not be able to equip any of the healing spells/abilities, while the Lancer (class) will be able to. (See now, where the class is better than the job?) On the other hand Dragoon (job) gets access to special abilities that no other job can use, so in a big party setup wherein you won't need those healing spells because you have people covering it already, you can afford to be a job instead of class to dish out even more damage.

They both have very equal pros and cons. And if you think that people are going to demand jobs for bigger events/battles, then you are probably correct, but does that mean it will be 100% required. No, just make your own party, but I can see large group events being job driven while small group events being class driven.