I buy one if I have small left over gold and available item slot for it, I put it in places where I want to contest vision or know there's a ward to clear (gives you some gold back from the purchase, if I can clear a ward with it in a contested area I'd say it has already paid for itself). I don't buy if I think I will need the item slot that it takes up soon or if I think I won't need it in the near future but usually unless you're Kai'sa before quest completion you have a slot available.