>a bit of fs corruption in e.g. /var that takes /home with it
That is a weakness in the filesystem that shouldn't require hard partitioning to resolve. It would make a lot more sense for a filesystem to be able to place files from different directory trees into internally independent allocation groups, such that a corruption in one of them would be no more likely to take another down than if they were in separate partitions.
That would be one way to radically speed fsck times as well.