I think EXP gain is quick enough as it is, and that this is also partly responsible for how we seem to have a lot of players who struggle at end-game activities.
If you spend more time playing a particular job then you will become more efficient with all skills involved and it will expose you to a variety of different types of fights. The more of this that you skip because you are doing FATE's to fill the EXP gaps or using as much rested bonus as you can, then you don't fight all that much and loose out on some crucial skill development.

Unfortunately people want to get to 50 quickly and there's isn't much reason to level up through open world content such as regular mobs as the difference in EXP from it and dungeons seems to be too large for anyone to take the long route to 50.

I actually hope that when they increase the level cap that they either really push up the EXP required or add some higher lvl mobs than the cap like in 1.0. Whilst I am against any kind of ridiculous grind that makes the game feel like a chore, i would rather grind a bit so i can feel some sort of reward from my efforts.