In the old-school cosmology, either you had magic to overcome the environmental "suffocate and die" conditions (in which case they became irrelevant), or you just didn't go to those planes.
No reason the Plane of Salt couldn't be something as cool as the Feywild . . . but it wasn't, was it?
It wasn't, but that is not the issue. They don't need to be cool. But there could be places we can't go to or don't care to go.
From a pure game design point of view - the quasi-elemental plane of vacuum is useless. It serves no purpose during play. So off it goes. That's at least the 4E take. And I generally like that take.
Another opinion is simply - it is just some background fluff. It adds a detail to the world.
But is it "useful" fluff would? Being able or interested to go there is not the only qualifier for that. The sun is not a place to visit usually.
But can you use it in a story? Can you use it as part of a historical reference, like
"The Great Archwizard Deben was the first to ever enter the Vacuum plane, which later proved his downfall - in his campaign against the Dragon Saxomirdosa, he regularly retreated there for his research. Saxomirdosa send him a curious gift through an agent - the eye of a beholder, frozen in time. The enchantment freezing the eye in time ended as soon as the Archwizard touched it, negating his magical protection against the hostile plane and killing him instantly."
Apparently, you can tell such a story. Though did it really require a vacuum plane? It didn't actually require any planes at all, the deep ocean or a magma sea would have been sufficient.
The question might be - too how many lengths do we go, how detailed do we get? If the quasielemental plane of Smoke has no more story relevance than a magma-sea, couldn't we just use the magma-sea if we need it and not bother detailing that plane in our cosmology? The magma-sea is even a given in our world!
Or maybe this is not the right question either. Maybe the question isdo th quasii-elemental plane of Smoke or Vacuum have their own story? The Feywild sure does. It has the stories of Eladrin, Feys. The Underdark has its drows. The Astral Sea has the gods and Hell. The Elemental Chaos has the Primordials and the Abyss.
Or another view might be very... "physical" thinking. How does the existence of the quasi-elemental plane of ooze affect the world? The sun has an obvious effect on it. The Feywild seems to have one. The 12(?) moons of Eberron seem to have an effect.
Something that has no observable effect on the world is the same as non-existing from this perspective.
Well, ultimately it's all a matter of preferences and opinion. I prefer a more focused cosmology - describe me the big, important parts of the universe, those with their own stories and that invite to create new stories.