The fact that Planescape was ham-handed about its incorporation of material from other D&D settings that wanted nothing to do with it is not bias, it's reality. The Dark Sun material goes out of its way to cut the setting off from the then-established Great Wheel cosmology, and the Planescape material just keeps reeling it back in.
Planescape didn't intrude itself on Dark Sun all that much as I remember. Yes, the two were officially connected, as was every other 2e setting, but Planescape generally said that connections to Athas tended to be rare and/or difficult to use. This besically kept the two setting from interacting too much, while leaving things open enough for a DM who might want to use some sort of connection. And for DMs like me who didn't play in Dark Sun, it didn't matter much, except as a curiosity.
Honestly, I think Planescape was less an offender here than Spelljammer was. Spelljammer, after all, had 3 accessories connecting it to Greyhawk, the Realms, and Dragonlance. Spelljammer wasn't really a good fit for DL, might not have belonged in Greyhawk, and only really fit in the Realms in very specific places like Halruaa if that.
Planescape though didn't have much in the way of crossover material like this with the exception of a FR adventure or two. Mostly when it talks about the other settings, it's about how they're connected to the planes and such like that, stuff that's useful for a DM that's looking to use it with another stting.
That behavior is the source of most of the resentment the community feels toward Planescape and why the setting is so controversial today. If they'd just left well enough alone maybe there wouldn't be such backlash today.
I suspect the fact that anything about the planes was shunted off to Planescape from 1994-1998 and official material saying "you need Planescape" for anything probably has something to do with it to. That and it kept rolling with the removal then renaming of demons and devils, though PS made some decent lemonade out of that -- "Don't use the 'd-words' around the fiends, they're racial slurs and it pisses them off (not like it matters if you're just going to kill them
)."
The biggest example for me remains the divisions between the Ethereal and its coterminous planes and the Astral and its coterminous planes. It has many consequences, but perhaps the most visible is spellcasting. Playing a wizard or cleric in Planescape is a nightmare (admittedly, an entertaining nightmare for some), because there's this arbitrary (in my opinion) split between the two halves of the cosmology. Ethereal spells don't work on the Outer Planes. Astral spells don't work on the Inner Planes. Elemental spells "work" on the Outer Planes (for some reason), but are /colored/ by the plane they're in. Every Outer Plane has its own list of spell school effects. Clerics lose an insane amount of power when traveling to the Inner Planes. Powerful magic doesn't work, full stop, in the "center" of the Outlands. And ALL of this can be hand-waved if your character is carrying the right "key."
This entire system is incredibly thematic, which is great; unfortunately it is also monstrously and unnecessarily complicated.
Probably that's the reason it didn't show up in the 3e MotP, it IS pretty damn complicated, and somewhat confusing for the DM too. It does help to codify some rules about how magic was affected by planar travel that appeared here and there earlier in the edition (at least the Wizard's Handbook had stuff like this). But that's less Planescape and more 2e's obsession with how unbalancing magic can be IMO, and the edition's various attempts to reign it in. Then 3e takes off the restrictions, eventually resulting in the godliness of Tier 1, and creating DM nightmares everywhere.
But that's not true. Nearly every roleplaying game in existence has a single setting and cosmology, which it expects all players to adhere to.
D&D is practically unique in that it /doesn't/. The core setting is just guidance for new players, because the designers know dungeon masters immediately set out to create their own worlds and cosmologies as soon as they've got their feet under them. That's been a thing since the earliest days.
Why the core setting infuriates people so much is a little beyond me.
Probably because of DMs and homebrewing. The game long gave DMs a lot of leeway in deciding How Things Work in each of their personal campaigns. When D&D starts throwing in its own setting concepts pretty heavily into core though, it creates certain player expectations for a homebrewing DM to deal with. That's why there was a lot of bitching over it in 4e, every time the rules make a specific reference to the background, either the DM has to find a way to shoehorn it into a setting (and a long-running campaign won't necessarily find it easy), or toss the stuff out and have to deal with players how think it's still there.
