I think the WetFire cubes must have something volatile in them, something that allows them to burn even in water. I imagine that when the packaging is compromised, the volatile compounds evaporate or sublimate away.
I've kind of decided that as much as I like WetFire that it isn't really practical. A tinder that isn't durable and dependable isn't really all that helpful. Maybe if I lived in a wet climate the hassle of constantly rotating stock and carrying it in a hard sided container would be worth it.
HJ
I think the wetfire cubes have some value but perhaps not in a soft sided PSK. I'm going to carry the tender that's in Doug's kit in the PSK with some additional wetfire cubes in my backpack and SUV. While the wetfire is impressive it wouldn't be if it gets damaged and won't do the job.