[Khumak]

The lack of backstab basically makes swashbuckler an "almost" fighter with thief skills. Great if you don't really want a thief but still want to be able to disarm traps, pick locks, etc. Swashbucker>Mage can work as sort of a substitute for a Kensai>Mage. Less offense but a lot more defense + thief skills. Makes a great tank.

I normally have 2 thieves. Kensai>Thief as my "late game" thief. Assassin>Mage as my "early game" thief. If they both dual class at around the same time you've got a fully functional thief for the whole game and between the 2 you've got a badass fighter, a badass mage, and 2 badass backstabbers.