Shaffer, you are right in one thing: You can put anything in the Elemental Chaos that you could in the Elemental Planes. Probably even more, since it's the elemental *chaos* and you can throw everything together as you like it. But which you use is a mattter of cosmology and setting assumptions. Are Matter/Energy as important as alignment forces in your metaphysics? If so, each should have its own plane of origin, unless you also have the "Ideological Chaos". These are the building blocks of the cosmos, and having them there, or not, and in which constellations, says a lot about what kind of world it is.
For example, in my homebrew world I use a simplified and modified Great Wheel with the three Transitive Planes, four Elemental Planes, two Energy Planes and nine Outer Planes. I don't use the "in-between" outer planes because I couldn't make the symmetry interesting, so I folded the bits I liked from those planes into areas on their neighbor planes. I thought the quasi/paraelemental planes were clutter, so those environments are now found only on the borders between the (finite, thank you very much) inner planes. Are those inner planes monolithic and inhospitable? You bet. Fire in the material sense is what the inhabitants of Elemental Fire *breathe*. They drink liquid flame and their structures are solid fire-stuff, in many different varieties just as Material beings build in brick and wood and glass. And I can hear the naysayers already: "Needless symmetry! Useless fluff! Fancruft! Arm-chair-DMing!"
And it's true, the way they define these terms, but so the **** what? I am an unrepentant armchair DM. World-building is one of my favorite pastimes. The ideas I base the homebrew on should be followed through consistently throughout the setting, or what's the point? If the layout of a setting demands areas to exist that normal PCs ostensibly can't interact with, then so be it. As one previous poster wrote, our world's physical makeup demands the existence of the inside of the sun. Of course we can't ever go there, but but it has to be there, or it wouldn't be the same world! So it might be with the Elemental Planes, or any other foreign plane, in an RPG setting. And why should the PCs expect to be able to exist there anyway? They are literally not made for that world, it should by rights and by all logic be alien and inhospitable. And if that's your taste, it allows for an interesting perspective on the setting when you run an Efreeti game, or it shines a bit of spotlight on the master pyromancer when he can go where no mortal has ever gone before, or it reinforces the otherness of the Azer character who can go where all the other PCs would instantly die, but can't have a drink of water.
Of course, a commercially published setting should allow for as many games and types of game as possible, and should appeal to a wide audience. Different setting assumptions from my homebrew are to be expected, and are probably a better choice for them. But don't say that there's no difference, because the cosmology of a setting strongly informs what the setting is like, beyond just "what you can have there".
