The player already has no motivation to host nobles what with the irritating mandates and all, thus the frequent arranging of 'accidents'.
I feel you may be only looking at the idea from a players prospective. If we imagine ourselves as that noble for a moment: We're a VERY important dwarf of course. So obviously we are worth protecting from all the havoc that may occur within a fortress, and it gives us a happy thought, having these burly dwarfs around to make sure a goblin doesn't stab us in the gut. Doesn't seem like a waste of a soldier to the noble does it?
I don't see any reason not to put something in game as it becomes available just because everything related to it isn't ready yet. Players will find uses for bodyguards, and assassins murderers etc. will show up eventually as well. As with everything, it's only a matter of time.
I have already told you definite reason not to add something pointless into the game, the newbies do not know it is pointless and so will waste resources on things that being newbies they likely do not have the resources to waste on.
Whether our noble thinks he is an uber-important inherently is dependent on his personality values for
[PRIDE]. Whether our noble thinks he is uber-important for being a noble is dependent upon his values in regard to
[POWER]. Thus a noble with lower
[PRIDE] personality and low
[POWER] values does not automatically think he is so important that he should deprive the fortress of a valuable soldier in order to protect himself against the next to non-existent threat of being killed at random by a vampire.
I was going to remark that vampires tend to target people who are alone in a room, which tends to be my busily working administration staff, nobles sitting in their grand halls 24/7, and high end craftsdwarves who are usually furnished with massive multiroom megahalls and who rarely share a room with their families. Everyone else is either sharing one room with their families, or is at work near other dwarves. As a result, vampires hit my top level dwarves in my forts much more often than not.
e: misc edits
A funny game you are playing. Nobles normally work like all the other dwarves, so why would they be sitting around in their rooms unless you specifically deactivated all their normal labors. They will also share their room automatically with their families like everybody else. It is also funny given how rare dwarf vampires are that you know so much about them, I have yet to see a vampire in fortress mode at all.