I have a multi-purpose kettle that I can use to boil water, heat the room, cook a small amount of food, or use as a sand battery for when its cold in the desert, where the kettle is designed to operate as long as there is a handful of material to burn.

It is fair to observe a separation methodology, but I also have to say, in some cases multi-purpose devices have their place.

If, say, the self-heating mug involved solar harvesting, I'd put a couple in my kettle bag, for sure.