I cook in bain-marie anytime I am cooking with milk without a thickener like flour, corn starch or something like that. That is a "rule" I made for myself. But also I remember my Mom cooking condensed milk at home in a heavy pan an placing a saucer on the bottom of the pan so the milk wouldn't overflow when boiling.
"We are what we repeatedly do. Excellence, then, it is not an act, but a habit." Aristotle