I know that people can get away with single layer boots in very cold temperatures when moving quickly (I’ve used single leather boots in Antartica and Greenland down to -25c when using Nordic skis), but the problem comes when you’re forced to stop moving. Say for instance your feet are not being damaged by the cold yet (as I doubt they would be warm - just not frostbitten) but your mate slips and twists and ankle, or gets AMS, or you run out of food at and time at 17,000 camp and have to come down in high winds. In such an situation, overboots or no overboots, there is a good chance you’ll loose toes, big chunks of feet, to even your life. I know of a guy who climbed the NE spur of the Droites in winter who lost half his foot due to wearing leathers, and the one day route turning into a multi day affair. Add in dehydration, having to sleeping with your big boots in your sleeping bag, trying to keep the sweat out of them, and the general lowering of the bodies ability to fight cold… and well, I think it would be crazy and fool hardy to go up the mountain with Phantoms. Sure Spantiks are not cheap, but compared to either being disabled for the rest of your life, or having a whole trip turn into a survival mission, they’re a good investment, being warm, solid, and good for multi day/week mountain trips. As with all such things it’s worth looking again at the blog I wrote earlier in the year about survivability - a word well worth considering when leaving the safety of a more sports specific performance mind set.