Carlos wrote: "About the disadvantages as a standalone garment well ... I suspect that is clear that this garments are designed as a undergarment."
Yes, certainly. But lightweight backpackers are always looking to minimize weight, often by system integration and multiple use of items. Since fishnet doesn't work well as a sole layer, we have to carry another layer that can serve that role in warmer conditions.
Jerry wrote: "Maybe it's better to have the sweat stay next to your skin until it evaporates, because that might tend to reduce sweating?"
I agree. Except at high exertion levels, if you are sweating, you are too warm. If you are too warm, you want the sweat to cool you. Having it evaporate off of your skin is a good way to do this. If you wick the sweat away from the skin, a) the wicking layer then insulates your skin from the sweat as it evaporates, and b) the wicking layer wets out more easily.
I also want to note that the fibers of a fishnet shirt, be they cotton, poly, or wool, still provide some wicking that improves next-to-skin comfort. But, they simply wick less than a more densely knit/woven shirt due to their much lower skin contact surface area.