I think some people, who've never experienced multi-generational living themselves, think of it in an idealized way, like the Waltons.
When she was alive, my FOO lived with my paternal grandmother, and it was NOT like the Waltons.
In some cases, where you don't have any active alcoholics, BPD's, control freaks, etc., it might be OK. (But how many cases like that do you know of?)