I meant loose non-wicking clothes allow the water from perspiration to stay on the skin WHILE IT EVAPORATES, thereby cooling the body. The problem I see with knit and wicking polyester or wool garments is that the evaporation takes place at the outer surface of the garment, but this cooling doesn't cool the body very much because the garment acts as an insulator, so the body just produces more sweat, which wastes water and salt and fouls the garment besides. Yes, if you open enough pit zips and whatnot, you get evaporation at the skin, but this is the same as if you were wearing something loose.
In cold weather, evaporating at some distance from the skin is thought to be an advantage, but it can also be a disadvantage, at least if you tend to frequently work up a sweat in cold weather while walking uphill or otherwise exerting yourself. Once a garment is sweat-soaked, there is the risk of mild hypothermia from flash evaporative cooling when you take a rest break. With a loose shirt under a jacket, you just take off the jacket and then it should be easy to stay cool enough to avoid sweating. Removing a base layer that is under a wind shell is normally much more of a hassle than removing a jacket and tends not to get done, so the base layer eventually gets sweat-soaked. Perhaps it is possible to avoid this with the correct choice and use of gear. My own experience is that even the thinnest wool base layer causes me to frequently sweat when I exert myself, even at sub-freezing temperatures. So I have to stop and remove the base layer, which is such a nuisance compared to removing a jacket that I eventually stopped using insulating base layers altogether. And then I began thinking about the whole base layer concept and concluded that insulating base layers simply don't make sense for most conditions.
Incidentally, my supplex shirt weighs 210 grams, which is just slightly more than what my old Ibex zip-t base layer weighed. Yes, I know that some supplex shirts weigh twice this, due to unnecessary pockets and other doo-dads.
Brynje polypropylene fishnet is the perfect insulating base layer, since it allows evaporation to occur on the skin, but yet provides good insulation when not perspiring (if used under a windshirt), and never absorbs much water. A Brynje fishnet shirt was the last insulating base layer I ever used, and it is truly remarkable stuff. Unfortunately, polypropylene fishnet picks up smells and these smells are hard to remove by hand washing, which is why I gave up on the fishnet.
I agree that nothing works well in hot humid weather, but loose supplex is probably the best choice (assuming cotton and hemp are ruled out) if you are forced to wear something, either for sun protection or for social reasons. Very loose cut, so you get plenty of breezes.