Uh, actually I wasn't really going anywhere with that point (which wasn't even may main point, by the way; just something I stated almost incidentally).
I was just stating what you just did: that being a meta-setting is exactly what made the Planescape universe so interesting, cause of the creative freedom it allowed.
Then again, I don't have a strong knowledge of it. I mostly judge it from what I've seen in Torment and what I've read here and there over time, but when i think about Planescape my first though isn't "Woah, it's so cool cause there's the Lady of Pain, and this weird city, and those specific planes with these weird things!" and so on.
What I think is more like "It's extremely cool cause one could really use this setting to build any kind of story, and you can have these portals opening anywhere at any condition, etc".