Wait...people actually use Raise Dead? I thought that is what preservation was for.
Indeed. However i tried to put useful spells also for strange situations. You can sometimes be killed in one hit or due to a special effect. (i remember in MM6 that some fountains can give you an instant death. I don't remember enough for the 7)
Also as Tolich said some creatures like titans can kill you whatever your protection, exept protection against magic.
In fact i myself rarely use preservation as i always try to keep the balance in hit points in my team. I rarely wait i got killed characters (maybe a reflex from solo games lol) and at that occasion i just use raise dead.
I generally only launch preservation for big battles so that, combined to regeneration my character can come back conscious after a time.