Why not THF job?

[Exstal]

SCH and SMN can answer your question, yes there could be enough skills to play the same class in a different way, while ninja looks more like a melee/mage hybrid Thief can be more of a melee oriented, with classic skills like sneak attack, I remember in 1.0 a skill that could exchange hate from one player to another, there's a lot of stuff they could add to make it feel different.

That can't answer the question。

SCH - healer
SMN - DPS

Unless you're saying;

NIN - DPS
THF - tank