What I have always heard is the DOT 5 (silicone) was developed for military use in the first place. For vehicles that sit unused for long periods, sometimes in harsh environments, then need to be fired up and driven right away, silicone fluid, which doesn't absorb water, made more sense than traditional fluid, which does.
Perhaps all DOT 5 does meet this milspec? It's not all a winning proposition though. DOT 5 in my MGA has a different, spongier feel, and even though it doesn't absord water, moisture is still getting into the system, because it's vented to the atmosphere. So instead of mixing, the water just sits there, unmixed, in the low spots, right? Not necessarily better, just different.