The problem with balance is that people put far too much emphasis on this...more than the game itself, in fact.
Yes, some DPS might be lower than others, or tanks can survive less or do less damage, but since almost every "usual" setup can clear anything in the hands of good player, it really doesn't matter that much. Moreover, since 95% of the content is done via Duty Finder, where you can't chose the jobs in your party, or static, where you probably play with people you know, and who probably have no issue letting you play the job you want, it's easy to progress with any job.

And yet, you can have some sort of customization while bypassing the "that build is the absolute best". I've suggested this in another topic, with tanks as a primary example. Create a MT build and an OT built. Since there is nothing to say that "MTing is better than OTing", and these two need lots of differents things, you coul at least have two buils depending of what the player likes to play the most.