Whips have low damage but have x3 melee range, are considered light weapons just like rapiers, do slashing damage, can be dual-wielded, and have a bonus to disarm. 1d3 damage doesn't matter for a rogue cause they do most their damage with sneak attacks anyways. The expanded reach and slashing damage alone makes it an excellent weapon for rogues.
(EDIT: the expanded reach would also improve the performance of the tumble skill and mobility feat)