its not as bad as you make it sound. It is basically a spot +4 damage -1 def tool to secure a kill. 25 nora for securing a kill is one of the cheapest options in the game and as bonus you get to keep the equip on your unit. It is just that a spell like poisonous fume (35 nora. 8 damage -1 def) is much more easy in use and so will likely be prefered to include in a bg, but poison fang is cheaper.
10 nora and just straight give Poison rank 2, so it works with ranged attacks and if you gain a rank some other way. Still worse than Frostcall because it doesn't change your damage type, but poison does at least give a debuff.
Always been this way. Was only ever run on occasion for things like skeezick sniper but even with that accounted for there have always been better equips for spot melee dmg accessible within faction.
For some reason I thought it granted a melee attack to the user. Even worse than I previously thought. I second giving it the frostcall quiver treatment (ranged attacks only, grant attack: poison, poison dot, cheaper) or something similar.