Keeping one open source on purpose, and I would recommend it to anyone.
A hen that is unwell, very young or very old sometimes will not use a nipple, and you will not notice quickly. A shallow open dish somewhere shaded costs nothing and removes the whole category of risk. It gets dirty, and that is fine, because it is a backup rather than the main supply.
Also useful in heat, when they will drink far more than usual and a queue at a single nipple becomes a real problem.