
The greatest problem of using alts is that you have to repeat the storyline (and the unavoidable click-fest cutscenes) with each one in order to unlock the features like the retainer (bank) and the chocobos. If you have only one character, you only unlock those once and you are done: you can switch from your level 49 Archer to your level 1 Gladiator and not only access your retainer but also mount your chocobo. Not to mention you can hold on some loot with your higher level class to outfit your lower level ones.
The second problem is that all classes benefit from cross-class skills, so chances are your alts will end up with three classes anyway (specially true if you want them to reach a Job, as those only can grab skills from other two specific classes, as far as I understand).
The greatest problem of using one single main is that you'll have to rely on leves, FATEs and dungeon runs (and to some minor extent, the hunting log) to level up, as you won't have as many quests available. However, because you get a bonus to experience depending on how much of a difference there is in levels between your current class and your highest class, you will level up faster.
Its second problem is that you are stuck with the same guy/gal all the time. But that might not be a problem at all; it's very subjective.
For all these reasons, I'd say that logistically ARR is not alt-friendly.
The second problem is that all classes benefit from cross-class skills, so chances are your alts will end up with three classes anyway (specially true if you want them to reach a Job, as those only can grab skills from other two specific classes, as far as I understand).
The greatest problem of using one single main is that you'll have to rely on leves, FATEs and dungeon runs (and to some minor extent, the hunting log) to level up, as you won't have as many quests available. However, because you get a bonus to experience depending on how much of a difference there is in levels between your current class and your highest class, you will level up faster.
Its second problem is that you are stuck with the same guy/gal all the time. But that might not be a problem at all; it's very subjective.
For all these reasons, I'd say that logistically ARR is not alt-friendly.