Claws were first able to be utilized as a weapon in Legacy of Kain: Soul Reaver (the first game in the series to allow the protagonist to be 'unarmed'). Raziel was able to extensively use his claws against enemies in Combat (see also: Combat (Soul Reaver)), where claws were faster than other weapons, but lacked much of the power. Raziel's claws, whilst allowing Raziel to perform several combos and effectively 'stun' a Vampire, were not powerful enough to finish them outright - instead Raziel had to take advantage of the surrounding environment to kill Vampires (using one of the 'finishing moves'), or alternatively impale the vampire using a Spear or other weapons. Raziel's claws were insufficient to permanently cleave and impale a vampire in themselves[Soul Reaver].
Claws are only used as a weapon in games which allow the protagonist to drop, break or otherwise lose their weapons; in both Blood Omen[Blood Omen] and Defiance[Defiance], the protagonist cannot lose their weapons.