For most Prime Material campaigns, I'd say we have way too many planes in general. There's only a limited amount of things you want going on, plane-wise, in a Material campaign. Some fiends, some celestials, perhaps robots or madness or something "alien," something to handle the teleportation and long-distance travel, something to handle the ghosts and things out-of-phase, someplace to stick the grand villain for eternity (or until his minions bring him back!), really just some basic needs.
Really, for most Material campaigns, there's a lot of virtue in just combining all the transitive planes, all the upper planes, and all the lower planes and presenting, like, three or four outer planes (a heaven, a hell, and a world "beside" this one, and maybe some untouchable things beyond it). And you really don't loose anything.
But planar adventures have long been a part of D&D, and Planescape gave us one of the first real attempts at a planar *campaign*. So now you have people who will rather justifiably feel that their favorite thing about the game is reduced to over-simplistic unusability and that their own needs were ignored if such a thing were to become core.
The planar variety of D&D is one of it's strengths, I'd say. There's an unlimited number of places to put stuff, should you want to. And if you don't need to, melding planes together is always easier than ripping them apart. Which is why I'd support two different "feminine wiles" fiends in MM1, and why I'd support the separation of the Ethereal and the Shadow in the core rules, but at the same time say, heck, no one is making you keep 'em separate, and if you don't run a heavily planar campaign, it's a good idea to join them.
So it's a good idea to join them. But not a good idea to present them initially as joined. If that makes sense.