My concern with this is that many of these creatures - daemons, for instance, but also modrons - existed before and independently of Planescape. I don't see why Planescape gets a monopoly on them any more than Greyhawk, or FR, or Eberron gets a monopoly on what orcs are.
I blame that on the way 2e did things -- everything had to be boxed sets for their precious campaign settings, you couldn't have anything generic beyond the splats. So you want planar material? Well, you don't get a 2e Manual of the Planes, nope
here's a big expansive boxed campaign setting so we can crank out 3 boxes on just the Outer Planes, a small box with a CD for The Outlands, then 3 more softcover books on the remaining planes. Then 3 more books just for the monsters. That's not counting the box about the Blood War, another book for the fiends, the player's book, several books on Sigil and the factions and assorted modules, because they're not as useful if you just want the information on the planes.* It's the same reason why the whole idea of bloodlines wasn't covered in a single product for a DM to just plug into a setting, it was combined with running domains (left out of the DMG when it was a typical high-level topic before like planewalking) into Birthright, or why al-Qadim didn't just get an Arabian themed sourcebook like 1e's Oriental Adventures, but instead became a subsetting in the king of 2e settings, the FR. Later 2e material like Council of Wyrms (well for a certain value of later; CoW was 1994, at the height of setting madness) and Jakandor did what this stuff
should have been doing all along -- presenting material in a plug-and-play manner that DMs could use in any setting of their choice.
The worst part? Planescape, Birthright, and al-Qadim all had some pretty good stuff, as did the other settings, which makes the fan base fragentation which we're probably still suffering with to some degree more tragic. All that wealth of creativity, and there've always been those who've hated it.
*I have most of this stuff actually and I don't regret getting it, but I'm pointing out just how absurd the campaign setting business model was. Doesn't help either that like I said in my last post the basic planar stuff should be more or less core, but from 1994 to about 1998 or 1999 (if not the edition's remaining run), 2e pretty much had only one thing to say about the planes outside the brief DMG descriptions -- "see Planescape". And it didn't help that some parts of it took several years to get printed (eg, the Inner Planes weren't detailed in PS until late 1998). Not surprising that players that weren't interested about the setting got annoyed by this. I don't agree with them, but I can't necessarily blame them.
The preference of [MENTION=22779]Hussar[/MENTION], and some other posters too, is to have only a light cosmology in the core materials. That's certainly one option. An alternative - for which I have a bit more sympathy - is a default cosmology, but in that case I want the most inspiring, engaging and easy-to-use cosmology, and for me Planescape doesn't tick that box.
That's the way I would prefer it myself, but that's where we start getting into edition wars. See, I
liked 3e's fluff-lite approach which presented stuff do DMs to use as they please, including not use at all. But some people didn't like that at all, so then along comes 4e with its background and cosmology, which doesn't necessarily mesh well with everything before (and as a result, FR gets blown up), and there are gamers that don't particularly care for thsat approach either.
Just because Planescape had a number of fans isn't, on its own, a reason to make it the default cosmology.
The problem is that PS is based on what was the default cosmology before the setting even existed, it didn't just come out of nowhere. So it already had up to 18 years worth of legacy by the time the first PS box saw print.
