I hope I don't betray any latent fascist tendencies here, but I read all this stuff about how horrible nobles are, but besides the occasional fit because someone had a better tomb than him, my nobles have been pretty useful. In fact, even that fit was a good thing. I had inadvertently placed an artifact coffin in the common graveyard. My duke was going on about building more buckets, then I realized that all my buckets were filled with muddy water, and therefore useless. On two separate games I had a noble who was the trader, and another the bookkeeper. In fact, I have a hard time getting the noble trader to the depot because he likes to give water to all the people in the hospital. So, up with government! Down with the common man! Hey... wait a second..
I started with DF2010. In 23a, which I have played, all nobles only performed some kind of hardcoded job, could not be replaced, and had ridiculous demands. In 40d, which I have not played, there were two kinds of nobles: those who were appointed by the player (manager, bookkeeper, I think the chief medical dwarf), and those who were not. The unappointed nobility arrived automatically when certain criteria were met, were fewer in number than in 23a, but were also less useful (besides the Dungeon Master) and still had ridiculous mandates.
In DF2010, a lot of the nobility became broken, so all could be appointed at any time by the player or elected by the dwarves except for the baron and the king. The baron would chosen by the player though, and you got the choice not to become a barony (you never actually got the choice to become a barony, because of bugs).
Finally, in DF2012, many of the noble bugs were fixed, but only the baron remains chosen by the player. Both the Baron and the King perform work in this version, even though they have "noble" under their status. Usually, the position jobs, if any, work alongside the dwarf's other assigned duties, which is why in your civilisations list it will note the leaders as "queen/fisherdwarf" or "king/weaponsmith". If you look at the succession game Moltenchannels, for example, the baron/count/duke spend most of the time cooking masterwork kitten tallow biscuits and other food.
The next version of dwarf fortress should deal with replacements of the barons, kings, and other important people if they die off.