I reckon that the nobles are pretty much OK as they are. It appears that they
do have skills, the trick is to find out what they are and balance them against the nobles quirks.
For example, my current Countess absolutely
loves trading. Every time my assigned Trader was busy doing something else and I hit the panic-button saying "anyone can trade" in the hope that someone, anyone, would catch the traders before their imminent departure... TaDah! She immediately rushed to the rescue. In the end, I reassigned my trader to the military and she looks after all trade now. And what does she ask for in return? Cabinets. And the occasional Gold item. Both are mandates I'm more than happy to fulfill as they're permanently in the job queue anyway.
And my Hammerer? He's a party animal and spends all his time attending parties at the well. In seven years he hasn't hammered
anyone, 'cos he's too busy mingling. So far my Fortress Guards have gotten to any miscreants first and simply escorted 'em to the cells. (Mind you, all his predecessors met nasty accidents. Finding my current Hammerer took some effort...
)
Sure, at first Nobles are a challenge, but
execution chambers natural selection soon culls out the worst and they become nothing more than a minor nuisance.