Ravenloft as it was presented in the first setting sourcebook was a demi-plane in the ethereal plane that could reach out and connect to other planes of existence. I loved that concept. So I totally get why people like planescape. And I do see the connection here. But I also remember how irritating Ravenloft could be to Dragonlance GMs, Forgotten Realms GMs, Dark Sun GMs, homebrew GMs, etc. For a Ravenloft GM, Soth was a wonderful addition to added to the setting (and got one of the better novel treatments as part of his introduction to the demi plane of dread). But I knew a lot of Dragonlance fans who could not stand that move and felt it cheapened the character by taking this break out villain with a great story and dropping him on universal monster island. I think a big issue is genre and tone, and when you start mixing settings too easily like that, it can create all kinds of problems for campaign flavor, and it also irritates people if their favorite character in one setting becomes canonical in another and has some kind of plane spanning journey.
I think what they are overestimating is how many people actually treat their D&D campaigns (whether homebrew or setting) as multiverse, and how often and easy multiverse travel is.
