Almost everything on CD pre-PS2 tends to come with a spine card, bar PS1 games which tend not to have them.
So if you were looking at a Saturn game you'd be safe to assume it should probably include a spine card, although there are a few that don't - but if you were looking at a PS1 game you'd more than likely be safe in assuming it wasn't supposed to include a spine card, apart from the few exceptions that do.
Clear as mud, right?
On the whole I view spine cards as a bonus - they don't do anything useful so I don't lose anything if they aren't there and when I do have them they always end up tucked inside the case so you can't tell which ones do or don't have them anyway.