I hope it's uncontroversial to say that D&D is a game about killing things (and, of course, taking their stuff). I fully acknowledge that you can do other stuff as well, but the primary purpose of the system is to support killing things.
This doesn't jive with my understanding of what's important about Planescape at all. I mean, yes - killing things is absolutely a valid thing to do but it's by no means about killing things. If I had to choose just one thing that Planescape is about it's the importance of belief/ideology and all the messy stuff that comes with that. It also could be about going to strange and wonderful places where lots of weird shit happens, depending on what angle you want to look at it from.
So the question is: How do you reconcile the focus of D&D with the focus of Planescape?
Or: How do you make your games about Planescapey things rather than about D&Dy things, given that the system is urging/rewarding/supporting types of play that are not this?
And, of course, feel free to tell me about how my understanding of D&D and/or Planescape is entirely wrong and how it's all quite compatible.
). If I understand your position correctly it's: 'The system doesn't have a strong impact on the game's content or themes, it's just there to handle the messy bits like combat - all the other stuff is up to the players/GM'?
D&D is a game system, it's a tool meant to support your story and campaign.
You don't follow everything they say about what you should do with a campaign or not, because all of that stuff in D&D is simply Guidelines and not directives.