Hi Eodman,
It is a back up plan for when the puritabs and the iodine runs out. I would use the distillation kit if I want more reassurance then just boiling or, like you said seawater.

The reason I would use the PSK tin is because that is readily available and (nearly, straw and plastic bags are) nothing is extra weight. Mess tins you pack in your rucksack. You might have to ditch that. Every thing I mentiond here fits in my (future) chest rig.
(Now you mention it I could even use my metal mug as the boiler and the tin as catcher, I can easily pour the water in my cateen from the tin, connecting it back without burning my fingers. The metal mug has a large capacity and the top can be covered with aluminium foil. At the most I’ll only lose a few droplets.)

Of course I need fuel for the fire but nature is a rich source.

Anyway I appreciate the points you pointed out.

Reinhardt