FFXI wasn't a perfect game by any stretch, but I learned a few things from playing that game for a decade:
- Some jobs were more sought after than other jobs for certain content
- However, nearly all jobs were useful in some content

What I mean to say is... you wouldn't bring MNK to Fafnir/Aspid/Behemoth since they would just feed the mob TP. But, MNK was a beast to gain merit points to spend on other jobs.

BRD, BLM and SAM were welcome in most content.

Healers and support (WHM SCH RDM BRD) were welcome in all content.

The thing is, in that game it was very hard to level a job to the level cap (before the game went easy mode). So the problem was that if you didn't have one of those "welcome jobs" you would be excluded. However, if you did have your "fun job" and then BRD, BLM, WHM, etc - you could go on your fun job for some content and then your necessary job to other content.

In FFXIV, leveling is so easy. They don't need to balance the jobs. Let's say RPR is not all that great in savage (just an example), just bring it for your daily expert roulette and tear stuff up. It isn't like you aren't going to clear roulette dungeons.

We are sacrificing too much for balance.