If they use the same content, then that's consolidating resources and expanding the playerbase of that content.
For example, if the Masked Carnivale stages 61-70 can be played as BLU and the next limited job, with seperate achievements and rewards, that's more longevity for the same content.
The fact that any new limited jobs need to be balanced against BLU is no more of a problem than the fact that the other 17 combat jobs have to be balanced with each other.
Remember, this is still side content.
Was Chocobo Racing a waste of resources?
What about Lord of Vermillion, Diadem, Ocean Fishing, Ishgard restoration?
Is everything you don't personally play a waste of resources?
There are a bunch of job concepts that are never going to make the cut as full jobs, Onion Knight, Mime, Beast Tamer and Puppetmaster are some of those non-viable jobs that could work as limited jobs.
Why should we get more very very niche side content like Mahjong or... Blitzball, when we could instead get combat related side content like Blue Mage?
I'd much rather have combat related side content, than them "waste" resources on more minigames like Blitzball, but to each their own.
Blue Mage I admit could have made it as a full job, but it didn't. Besides, we're not likely to get more than just one more caster DPS in the games lifetime, and there's already calls for Geomancer, Time Mage, Onmyoji/Oracle, Necromancer... so only one of those is going to make the cut anyway. Adding BLU to that competition wouldn't have helped much and it's aesthetic is too similar to Red Mage anyway.