A pan will boil water. A kettle does it faster, because it is the right shape and has a lid that fits properly, and on a finite gas canister that difference accumulates over a fortnight.
The real argument is a practical one though. Boiling water is needed constantly in this cooking: rehydrating couscous, dried mushrooms and milk powder, treating water, washing up, and tea. If all of that goes through the one pan you cook in, you spend the trip washing the pan before you can boil, and boiling before you can cook.
Whistling is worth it. An unwatched kettle on a camping stove boils dry faster than you expect, and a dry-boiled aluminium kettle is scrap.
Stainless rather than aluminium if it might go on an induction hob, and either way check the base is flat: a slightly domed base on a narrow burner is unstable with two litres in it.
If you are cooking on wood rather than gas, the Kelly Kettle does this job far better and needs no fuel you have to carry.