I can't speak to endurance (I got into IF after my endurance-athlete days) but I know that body weight-based athletes do this to perform better. My experience is specific to rock climbing.
When you are right at the edge of your abilities being just a couple pounds lighter can feel like a world of difference (people hone down their rack to precisely what they need for a route, nothing more), especially on sport and boulder problems. My anecdata points to it being great for the project/performative aspects of the sport but as a tool for consistent progression/improvement, I am less confident in stating anything.
It is an option but it is neither particularly breathable, nor flexible, nor inexpensive.
For some PPE all you are looking for is no-melt (which would leave molten goo on your skin that would increase the damage)or provides a minimal level of protection, e.g. NFPA 70E PPE level 1 for arc flash, suitable to your expected risk exposure.
From a former climber's perspective, moving expediently through terrain is frequently the safest mode of operation. Every second you are in the mountains you are exposing yourself to increased risk of a catastrophic accident. Even if this is a terrifically low probability event if you spend enough time there it will likely bite you. Thus pursuing the fastest methods of travel is, from my perspective, aspiring to the highest ideals of mountaineering because it allows you to do it for longer (hopefully).
When you are right at the edge of your abilities being just a couple pounds lighter can feel like a world of difference (people hone down their rack to precisely what they need for a route, nothing more), especially on sport and boulder problems. My anecdata points to it being great for the project/performative aspects of the sport but as a tool for consistent progression/improvement, I am less confident in stating anything.