well if skill awoken, she becomes more reliable, not depending on an assassination gives you a good mesure of whether or not you can kill an enemy or even clear a map; that, and her skill gains 20 extra seconds and goes from 1.5x attack to 1.7x attack
and to be honest if i want ranged assassination i would just throw saki, Sybilla has more important roles to fulfill as a meelee magic attacker :P