At first this sounds like a joke, but it makes sense. With padded shorts, sweat can be dissipated before it gets to the saddle, but if the barrier between the skin and the surface of the saddle is minimized, then a soft saddle is more likely to absorb the sweat. If the surface of the saddle was a complete vapor barrier, then sweat would not readily dissipate and evaporate, which might keep the saddle clean, but would make for damp shorts if the saddle was soft and squishy. Leather (being skin, after all) is chemically more compatible with sweat than fabric or plastic, and it breathes. If that's all that is, the only issue is how to deal with chafing. But if it is wrapping some kind of foam padding, the padding will eventually absorb the sweat vapors. Eww.