The article is right on every accord, in practice it's rediscovering warm water...
The poll result... the vast majority wants a default but they are split between the 3e and 4e defaults, however the vast majority also want plenty of examples. This really tells me, that there shouldn't be a default. Otherwise, those parties who wanted a default are going to fight each other again because one of them "won" over the other.
People should realize that "default cosmology" almost only has cosmetic meaning that are always easy to adjust to your needs, and as such it's not worth fighting over it.
As Eberron and other settings illustrate, planar cosmology can have a fundamental effect on the material plane in some settings, influecing campaigns right from first level - it doesn't need to be something you leave until higher levels to implement.
Story-wise it can have a truly massive effect, but mechanically the only effects are on how a small bunch of spells work (typically travel spells using the planes), but normally all of them can be "fixed" by changing their description.
For example, who really cares if a "Shadow Jump" spell used for teleporting between two shadows
really takes the caster through another plane called "Plane of Shadows" or achieves the effect in another way, not even necessarily fully explained?
Because of that, you don't
really need to design your cosmology until you actually use it, i.e. until you take your PCs to other planes. And even then, you can still changes.
A published setting is another matter, of course they want to decide the cosmology early on, just like they also decide the world layout (which is by the way another example of something you don't
really need to design beforehand).
So no, creating a planar cosmology is not something that only well-practised GMs will do, and even if it were, the rulebooks are not purely a beginner's guide - they can, hopefully, provide useful advice for more experienced players as well.
Yes. Designing a cosmology is dead easy, it doesn't take a well-practised GM.
Manual of the Planes-types book don't so much provide necessary "how to", but rather inspiration, ideas to steal, tricks to make planehopping interesting etc. As such, they are certainly useful to experienced gamers too!