Ah, but we're discussing the 3I, where nobility carries with it a bit more of a traditional role than just figure head. That's kind of what I'm getting at. Nobles were administrators of public and military (local) policy. A knight of one province may "make war" with another knight of another province in the middle ages on a whim (say, "I like your vineyards and think they should be long to me"), and given the autonomy given to worlds of the 3I one is left to speculate if something like that doesn't occur on occasion; i.e. "wow, your megacorporation has a plan for a new fangled device that'll cut a financial swatch in the Droyne Device-X department...think I'll organize a strike team to run a smash and grab operation on their corporate R&D so I can get in on the action...").The Imperial Nobles in no way are shown to have military rank by virtue of being nobles.
They are shown to be in the military chain of command as civilian governors of imperial regions. That is, a sector duke is in the chain of command of all assets under the sector fleet (sector naval HQ), in the same way that a state governor is in the chain of command of a US National Guard unit... he's not in the unit, nor even the service (usually), but has authority to issue binding orders by virtue of his office (not his title).
For a modern US example of Title vs Office - Governor Knowles, former governor of Alaska, is entitled to the title "Governor" for the rest of his life, having completed his term of office. He can't use "Tony Knowles, Governor of Alaska" as that form combines title (Governor) with office (Governor of Alaska), and his etiquette people don't let him use "Governor Knowles of Alaska" as that form is used only for the current office holder. AKARNG personell still render him a salute, for his title, but don't take orders from him any more, because orders have to come from the office, not the title, even tho' the title is earned by election/appointment to that office.
Gov. Knowles is comparable to a typical honor noble in the 3I - rates the honor by title, has no authority because of no office.
Gov. Parnell of Alaska, current governor however, can give orders to AK NG troops and airmen, because he holds the office of Governor. He's akin to a sitting Baron of a world in the OTU. He can (and has) asked for active duty forces to be seconded to civil uses (specifically, MP's to assist state troopers on specific occasions), and the JBER commanding general did so... not because he had to, but because he found it to be of benefit to both the state and the nation, and could do so without significant compromise of his mission. (They assisted the troopers with a sobriety checkpoint on New Years.)
President Obama could countermand any such cooperation, if he had reason to, as could the Pentagon; likewise, the president could nationalize the NG forces, and take them out of Gubernatorial command (as was done in the 60's in the US South) - the equivalent would be the sector duke or his warantee giving explicit orders to ships or troops in the subsector fleet, or to colonial units on a baron's world.
From that, to something more mundane like cracking down on a protest of Sir Jerry who just raised paper and e-storage taxes to fund some program. If Sir Jerry is part of the government, the depending on the government, Sir Jerry might or might not order his personal guard onto the street to bust some heads.
Remember, Hussein's Iraq, by the books, was a "republic". The fact that he set himself up as a dictator (or "king"), is beside the point. His word and actions were to be obeyed, or you were shot, or worse. He was, in spite of a parliament, the "archduke of Iraq", as it were. His sons, even though they were not in the military, were also nobles (vile as they were). They were subject to arrest, but got out of it, and because of incompetence, did not get involved in military matters (much anyway). If either one of them had served, then guaranteed they would have had sway in Iraq's army and air force.
It all depends on the will and ability (mostly ability) of the polity to enforce and put safeguards in the political structure. If they're half willing, then the power structure has more influence and sway over them, as opposed to "the people" / constituents having command and say over their leadership. I think that's the key issue for how much a noble can push the envelope from world to world
