I know it's supposed to be risky, but I expected it to be risky because it opens me up to attack, not because I can't control where some of my damage goes.
It's both.
Most poke supports are poking with some kind of ability that could accidentally snag them a minion and/or they are auto attacking at not the greatest range. The benefit is of course doing damage and making gold.
Think about champions like Janna, Karma, Zyra, Lulu, and Annie. They bring a lot of utility and damage, some more than others. If the item had no cooldown or a short one there would be no reason to not just spam your ability all day, poking, getting free damage, and lots of gold. It would get oppressive for the enemy and less impactful for the player, you. The extra damage from the item is best felt on turrets and during your all ins. If you ult, attack and Q and happen to kill a minion you either got 2/3 of your bonus damage or all of it.
More damage when its time for plays
More risk when its more passive/harassing/farming.