These could have their place on certain trips. Either for heating up a meal, cooking some scrambled eggs in a bag, going stove-less on a short trip etc. You could rehydrate a meal ahead of time and dump it in the heater once stopped. Not sure that I will spend the money on them without a plan but, it's a thought.
Oh, don't stick one in your back pocket on a rainy day.
“The more you know, the less you carry” Mors Kochanski
Sounds like the water is toxic and so you would have to carry that out. That makes it seem like a heavier alternative to a small stove. I could see it as useful for situations where you don’t want to risk a flame or fumes such as cooking indoors after a natural disaster or inside a tent.