It's more of a legacy thing, the gimmick of Onion Knight is that it has *dogwater* stats from level 1-89, then starting with level 90 and up, its stats start to massively spike, giving it perfect maxed stats at level 99, it's an example of Magikarp Power that is older than Magikarp itself, it goes a step further in the 3D version, where Onion Knight is a secret job and thus given more tools, including full access to White and Black magic.
People had good experiences with Onion Knight, thus they want Onion Knight in the FF MMO, that's it.
Not exactly, while Onion Knight is the starting Job in the Famicon and Pixel Remaster versions, in the 3D version Onion Knight is a secret Job, owning to its legacy in original FFIII, Freelance is in turn to fill the role of "starting job" in place of Onion Knight, and unlike original Onion Knight, Freelance is given no special characteristics.
