Especially in a world like Eberron.
Example: in Five Nations we read that King Boranel's elder brothers were kiilled while fighting on the western frontier, and King Boranex "took his own life in grief the following month".
Uh?
Couldn't he find a cleric to resurrect them?
Only if he had the bodies, and only if the bodies could be returned in a time frame that 'Raise Dead' was viable because 'True Resurrection' probably is so rare in Eberron that there might be only a few clerics capable of casting it in the whole world.
Also, on my world there are usually prohibitions against raising the dead from royal families because it tends to create succession crisises. You can have a situation where the King dies, the heir is crowned King and then the old King raised from the dead. So now who is the true King? You can get civil wars from that sort of thing, so clerics are reluctant to intervene and in many countries prohibited by law and tradition from doing so.
Even in countries were they aren't, you run into this problem...
The same goes for Boranel: in the same page (57) we read that his wife has been assassinated, and that he offered a "sizable bounty" for the killer.
There are lots of ways to assassinate someone in D&D - even without inventing means - that will render resurrection impossible. We must assume that any assassin paid to go after the wife of the King must have the skill and means to actually utilize a method of killing someone and keeping them dead. In practice, it's easier to 'assassinate' a royal by methods other than killing them that merely straight up death:
a) Polymorph - One of the most accessible ways to rid yourself of a person in D&D is a baneful polymorph of some sort. You don't kill the King's wife, because that's a problem that's too easily dealt with. Instead, you keep the King's wife alive and hide her somewhere she can't be found. So long as the body is alive and undiscoverable, the King's wife is as good as dead and no high level spellcaster can do anything about it until the King's wife current form and location are ascertained. Ideally, you put the King's wife in a form she can't escape from and put her somewhere she will be safe until such time no one cares whether she comes back.
b) Trap the Soul - It requires higher level magic, but the absolute most secure way to kill someone in D&D is to trap their soul somewhere so that even if the body is discovered, 'Raise Dead' or even 'Speak with the Dead' offer no help. The serious players in world politics will be using methods like this to assassinate their enemies (probably in combination with several others on the list). Likewise, even wish is of minimal utility here. Not much can be done if you do it right until you find the object or place the person's soul is trapped in.
c) Lose the Body - It's not fool proof, because a dead body is an object and thus relatively easily found, but most resurrections require at least a portion of the body to be present before they will succeed. This makes for a 'low tech' solution to problem of high magic, in that if you dispose of the body well enough you may put the person beyond aid. The lowest tech form of this is simply have something eat the body that does a really good job of elimenating it - slimes are good at that, but it can involve burning the body (lava pits are good) or simply just hiding it someplace really inaccessible (depths of the ocean). There are of course some 'high tech' versions of this for those with access to magic. Disentigrate is an obvious choice. Less obvious but perhaps even more effective is hiding the remains on another plane of existence.
d) Foil Raise Dead - Raise dead can be thwarted by using an undead creature to commit the assasination or by using death magic, as can merely successfully hiding the body for a few weeks. However, such methods can be defeated with higher level versions of the spell like resurrection. Raise dead is no good if the Prince dies immediately on being brought back to life. Magical diseases like lycanthropy persist after ressurection and some resist being cured by readily available magic like cure disease. While nothing in the cannon directly does this, it wouldn't be hard to imagine a magical poison which resisted attempts to neutralize it. Until the poison is nuetralized from the body, it's no good ressurrecting the Prince to life. While such poisons ought to be rare and have an epic origin, they fit well with fantasy literature. For example, the poison used in Stephen King's 'Eyes of the Dragon' seems to have been chosen precisely because it was this sort of uncurable poison. For that matter, the entire fantasy cannon from Faerie Tales to Harry Potter becomes easier to explain if the deaths of important persons have to be done in special ways to prevent the person from coming back or being brought back to life. Why are there so many Princes in animal forms out there? Because someone recognized that for all the problems with the approach, it was harder to deal with being cursed and polymorphed than it is to deal with someone merely being dead.
So you see, in a world where it is well known that being stabbed through the heart is something your victim can recover from quite easily, assassination will definately mean some technique that takes that into account.
