As it stands, there really is no point to the varieties of poisonous mushrooms in the game's current version.
Adding the ability to take these mushrooms, boil them down and synthesize poisons/medicines from them would validate their existence in game outside of only being used for composting and multiplayer jokes.
Poisons could be used to tip/coat weapons and arrows for an added punch and much needed flare to base-game combat.
Certain poisonous mushrooms could be used to make tea/medicines for powerful healing buffs.