Sayings like "Cotton kills" is in my opinion replacing judgment with a rule. It would be more accurate to say "improper use of cotton kills". In many warm conditions, cotton is preferable. Last time I went up Taboose Pass in the Sierra (6,000 feet gain in 90 degree heat) I used soaked cotton as a cooling system.

But depending 100% on cotton in most mountainous conditions is not a smart idea. But as one piece of a system of clothing, I think cotton is at times very suitable. I usually take one fleece layer, one wool layer, one down item, and one light mostly cotton lightweight undershirt (simply for the comfort and the fact that it is cheap and does not stink like synthetics). Cotton certainly can be a PART of a layering system.

Bill, have you ever tried pack cloth nylon for pants? I would not bushwhack in jeans (not a tight enough weave). I have also seen really tough pants used by tree climbers/trimmers that are very tight weave canvas. I probably do not bushwhack in as severe brush as you, but I find wearing knee high pack-cloth gaiters works well.