Not that SE hasn't ever introduced broken mechanics before, but this one isn't very hard to counter.
Rank 30 in class opens quest for level 1 in job
Rank 35 in class opens quest for level 2 in job
Rank 40 in class opens quest for level 3 in job
Rank 45 in class opens quest for level 4 in job
Rank 50 in class opens quest for level 5 in job
Taking your example, ranking one class to 50, and the rest to 30. You have a couple of jobs at level 5, and the rest at level 1. Better go rank the rest of your classes to be able to unlock higher levels in your other jobs!
(Presumably, each new level in a job opens up more of it's special abilities. So, playing as a level 1 job with a rank 50 class would be kinda gimp compared to level 5 job with rank 50 class.)