I learned the hard way that using a job in a low level dungeon leaves you with next to nothing skill-wise, as the level-sync annihilates all your progress above a certain level. I can imagine that some people err on the side of caution when they first get their soul crystal, for that reason alone.

I did the Thousand Maws of Toto-Rak (lv.24) as a lv.33 Paladin. That was fun.