Well, the thing is, one of Rogue's signature abilities is Hide and the class uses a pair of daggers as weapons. That fits Assassin/Ninja quite well.
I feel like a Red Mage/Swashbuckler would be more of an in-your-face type of duelist rather than someone who would be sneaking around. And if you're sneaking around, you have to be pretty slick in my opinion.
