Rogue, while it was meant to be named "thief", covers a different path than the traditional Thief class imo, being more like shadow-police than people who actually steal things. In terms of lore, adding Thief to the game would fit well in Ul'dah, where the rich seem to keep the poor down. That's an example, but it would work well in Ishgard as well I think.

Rogues could have just been "Assassin" and there would be little difference. A thief, on the other hand, would represent a more robin-hood style approach. Of course, there are bad thieves who steal from any easy target, but that's what would set the Thief's guild apart.

Soon there will be two knights who use swords and magic of some kind... Thief could wield a single dagger, holding it upright or something. These things are possible and worthwhile if you ask me, a bit of creative thinking could set them apart from ROG/NIN fairly easily too.