It isn't for my use case. I have a notebook with journal entries, sorted by date (most recent, to latest).
Another notebook example contains personal notes. With a custom sort order, because I've a bunch of notes in there I use frequently, so I've organised them properly at the top. There's lots of other notes in there too, so not using a sort order is a pain.
I use these notebooks every day, and having to continually change the sort order is really annoying. I think this feature should be added since it's pretty straightforward to implement, and would remove a major hassle for users like myself (even if there's only a few of us).