a wonderful idea i saw someone else suggest was to bring him back after adding in the tavern/town stuff to fortress mode.
the hammerer is a dwarven justice enforcer, that does not deal with spys/theifs/etc of other races, those are simply thrown in the fortress dungeon, where the DM takes over interrogation/punishment.
considering toady's eventual plans for the stuff to allow your fort to interact with the world, he would have a pretty major impact on the political and criminal aspects of your fortress.
i just don't think having a immigrant noble be required to do something any other dwarf could do is a little silly. mechanics are full well capable of maintaining traps, and siege engineers know what their doing, also a 'dungeon' would have to confirm to the games idea of a dungeon, not the players.
on the same idea, adding a 'head' noble to each 'department' would be nice, particularly if he provided some management related benefit, NOT some magical superpower.
i.e.
Chief Engineer: mechanics and siege operators/engineers, helps with targeting, maintaining traps/mechanics and artillery.
CMD: already in, just fits with the idea.
Head Mason: masons/stonecutters, manages the workflow of his section, making sure everyone gets jobs they are well suited to.
Head Smith: obvious who he controls, can manage work orders of smithing stuff, and who is using what workshops.
DUNGEON MASTER: depending on his position could coordinate animal training/retraining, or in the case of him being the 'true' keeper of the dungeon, he could maintain the jail/dungeon cell assignments/releases/maintenance.
with similar positions for each 'area' of fortress life. the heads could also bring the demands of various members to your attention, such as your legendary metalsmith would like a nice room close to the forges.
having these nobles would also give the player more control of the workflow, such as only using granite for furniture etc etc.