I love Eberron's take on the planes, and consider it a complete cosmology in and of itself, and having it integrated into a wider generic cosmology detracts from that setting's unique mythology.
And without the default, everyone who wishes to use the planes/cosmology needs to produce a not-so-mini-setting book from scratch. Way more work for everyone instead of a few.
As for "fluff-lawyers", there's always the old addage: Don't play with d**ks. Bad players are no excuse for leaving a relevant portion of DMs with no structure off which to build or use whole cloth.
Not so
For me, the primary reason to have a cosmology set up has nothing to do with setting up an adventure -- it's for the PCs to figure out what/how they can interact with/take advantage of the "stuff" in the game world. A strong secondary reason is to act as inspiration for situations and NPC plans/actions.
Are there nearby transition planes like the Ethereal that allow covert intelligence gathering? Do some effects (bodily functions, ageing, poison, curses, divination magicks, etc.) cease on some planes and can the campaign movers factor that into their plans ? How easy/hard are the planes to traverse? Are there specific dangers stories speak of for traveling away from this world? About how difficult is it to "break free" of the base plane? What known magic exists that deals with planar traversal?
A default cosmology grants the basics for these answers and has me consider consequences and implications for the game world that I then use to introduce and evolve situations. The use of a default cosmology also gives me one less area I need to brief players on for knowledge the characters would have since I can tell the players to RTFM (or at least limit the briefing to what changes I've made to the default).
Very much so, you and your blanket "nope" and what-not do not make you right (and are dismissive and snide), many early DMs may be overwhelmed by the planes, a default could help
Sorry about sounding dismissive and snide, that's not particularly my intention. Although I am trying to say that I flatly don't buy the point that newbie DMs need default fluff, so I suppose it may sound that way. I can't say I've ever witnessed such a thing. Can you describe for me what an early DM that is "overwhelmed by the planes" looks like and how he got there without somehow choosing it for his game?
Having default answers for all of those narrows (or can narrow, depending on how much its burned into the rules) playstyle...which I thought was bad for a "unity" edition.
Also, how are "inspiration for situations and NPC plans/actions" and basically your second paragraph not a part of your adventure set up? I mean, I suppose we could sit around musing about such things all day, but unless they are part of an adventure or gameplay what difference does it make?
I'm not sure how "someone did it for me" has you considering consequences and implications for the game world. Otherwise, I'm fine the rest of it...in a setting book or boxed set. Just hand the players the Player's Guide to <X> instead of the MM, PHB, and DMG.
Because typically I find I'm a better editor than originator -- as an originator I tend to develop blind spots toward the creation whereas as an editor I can look critically and more holistically at the piece for consequences that will impact the game world.