My qualm with this line of thought, is every time I go into Masked Carnivale, I think back to Kingdom Hearts and the Colosseum side content (Later in FFXIII-2 also) which is arguably some of the best side-content for a game.
With my immediate thought being... "Why couldn't this be something that ANY job can do as side content?"
Like, imagine how much fun it could be to have a Colosseum side content (Which we have precedent for with the Coliseum in Ul'dah which currently is only used for PLD quests) where you get to fight bunches of different enemies, including multiples of (Less powerful) bosses. Like fighting 2 Ifrits. Maybe fighting Susano and Titan at the same time.
Since, I personally fail to see the exact reason why such side content, must be relegated to dedicated "Limited Jobs" instead of being available to ANY job? I mean, you can't even argue that it's because of how "OP" the Limited Jobs are (Supposed to be) due to the fact that in order to make it fun, it'd have to be balanced to their power level anyway (Hence why most things in the Carnivale are immune to insta-kill spells)...
In addition, there's also my inability to see why, if such content HAS to be relegated to a specific job... Why said job needs to be "Limited" in order to have that content? Since again, the content will be balanced around their power level irregardless of if they're "Limited" or not. So, what's the major difference between it being balanced around an "OP" Limited job and being balanced around a normal job variant of the class aside from one inherently removes potential content from people who aren't "The most casual players" (With the other not actually removing content from these players)
Like, a non-"Limited Job" BLU could still have Masked Carnivale. It could still have the ability to go around the overworld and learn cool new spells from monsters. It could still do all the same things that the current "Limited Job" version could do. Only, it would also be able to be played by other people who want to do other things with it too, it could be used by all players (Casual included) to do MSQ's. It could be used by raiders to raid. It could be used by all players (Casuals included) to do roulettes. It could be used by all players (Casuals included) to do Exploratory Missions.
This is what I don't fully understand. No-one has yet to present a compelling argument about why a "Limited Job" system is beneficial in any way. The best I've seen is about development time in regards to "Balancing", but that needs to be done anyway for the unique content they get used in because otherwise stuff like Masked Carnivale wouldn't be very interesting at all (For example, it's already markedly less interesting because of Final Sting/Self Detonation cheese, imagine if Doom/Tail Screw would work, or if things like Ram's Voice > Dragon's Voice combo was dealing "OP" levels of damage to everything in there)