I can put my metal kettle on the fire or in the fire to heat water. Can't do that with a plastic kettle or pot. I made allowances for space when creating my kitchen so that would not be a reason to have a fold up kettle.. I bought a fold up vessel to replace my SS pan for water (SS pan, 99 cents at a thrift store) but took it back when I discovered it could not go on the heat and I would have to heat water in a separate container to use the fold up sink, so twice the items for one job. Made no sense.
I no longer back pack so weight is not an issue.