I am just a simple guy, with no authority on the topic, I don't want to act like I speak for anybody else but myself.
I was really looking forward to Blue Mage. Finding out that it would be a Limited Job did disappoint me at first, but then I gave it a try and enjoyed my time. It was this overpowered job that could overcome tougher enemies than any other job at that level. It gave me the impression that I'd be able to use it whenever someone wants to do some old content, and it would give me some advantage. OK, sure, I would never be max level, but hey, maybe I could serve the role of a tank! Maybe for some players, this would be the chance to play a healer when needed. A jack of all trades, master of none! However around the level 50 point, it all stopped. My character doesn't feel that much more powerful than the normal jobs at that point, a lot of the special tools in BLU's toolbox are not that useful against the enemies, and to grind out those primal spells, I'm using the help of some high level players.
The Masked Carnival doesn't really seem to be made for me either, considering how much it is about precision, timing, which I'm not too good at, and even once I watch a video guide on how to beat it, I feel unsatisfied. True, I appreciate an alternative method of earning Allied Seals, but once I get all the riding maps for the ARR areas, I won't have any reason to continue this.
I see this as a problem. To me, the solo content doesn't last for long, and if Blue Mage were to be used in content with other players, then it should either be in that slightly overpowered state, which would justify it not being allowed in with the other jobs, or if it is to be balanced, then it should be allowed in with the rest.
To me, that is the critical issue with Blue Mage continuing on, because while you could just add more solo content, it would be just a short-term win for a niche player group. It would need to join groups, either justifying its separation, or not being separated.