Belief, as it turns out, is the key word there.  Planescape, unlike any other setting I am aware of, is a place (or many places) made, manipulated, and unmade by the beliefs of myriad sentient beings.  This mutability applies even to its denizens — the same people whose beliefs shape the multiverse can themselves be believed out of (or into) existence.  In such an exotic environment, wonderful and unique stories can flourish.

Does Planescape need the arbitrariness of the good/evil and law/chaos axes or the inhospitability of the Inner Planes to exist?  No, but without them, it would not be Planescape — just as Forgotten Realms without the Sword Coast would not be FR.  Contrived as it may be, the Planescape cosmology makes for great storytelling, even if some of the areas are never actually visited.  Starting from themes that its players knew well — the alignment system, the classical elements, various mythologies — the designers of Planescape created a multiverse that was familiar in some ways yet fundamentally alien (and mysterious) in others.  They did this without resorting to some of the most overused fantasy cliches (aloof elves who live in forests, anyone?).

You can have your Shadowfell and Elemental Chaos.  I’ll take Sigil and Avernus any day.

(On an unrelated note, Firefox seems unhappy about the word “inhospitability”.  I submit that Firefox can pike it.)