Iconic THF AF : We saw the exact Thief outfit in the trailer
Thief abilities : Looking at the description, Rogue already has some of these abilities. But I suppose even if Rogue has every abilities of past Thieves, it's not enough.
Lore : Classes have lore too in case you didn't follow the class quests
Job stone is not a gameplay mechanic, and the benefits from the Ninja stone will only enhance the Rogue

If people don't bring class in endgame, it's because jobs have everything a class has to offer AND more.