A mass use poison can't target as effectively, for one. A genetic-bioweapon could be dropped on an enemy or even neutral city and only kill the 'enemy group' without harming others in theory, that can be advantageous in many scenarios. It also inspires significant dread if the enemy is aware the weapon exists.

Second there's always the possibility of tying this to a virus as well. A perfect bioweapon could be largely carried without symptoms by the general population around the world yet be lethal to a specific ethnic group... With possie deniability about its origins.

These things do seem like they'd be better used for bioterror than as part of a country's WMDs, but if you look at how much shit the US tried to develop before that we know of that only a madman would want, I think it's still plausible they'd want it just in case.