Now, there's no denying that this makes a wonderful plot device for supplying even a down-and-out noble with forces and cannon fodder in general, who are willing to do the difficult or impossible in aid of the noble.
Andre Norton makes good use of it in Ice Crown, where Nilas Imfray's men break him out of his cage where he's exposed for punishment for treason. She gets the motives too. One man even banters with him afterward that he was just doing his duty toward his commander; Imfray returns that he must have heard the proclamation as he was shoved into the cage; the man claims to have suffered hearing problems ever since Imfray saved him from a rockfall. (Imfray solemnly tells him that bad hearing can take him out of the force, and the man assures him that if word got back, it had, it had.) And in Forerunner Foray, even though one loyal man tells "Turan" that all his most loyal forces have been sent away by his widow, he still has some; one offers a cloak against the weather because he had been in the army when they broke the enemy, and another, clued in by their odd behavior -- as they are, in reality, a pair of tomb robbers -- assumes that "Vintra" bewitched him and tries to come to his aid. ("Vintra" uses psychic powers against him, which is only confirmation.)
Michael Flynn's Spiral Arm has two sets of loyalties. Out on the Periphery, we have the Ardry and his court, with his agents the Hounds. This is the less formal version; in some respects, it resembles Dark Ages Europe. In the Confederacy, however, the Shadows are based on late medieval noble society. Full of elaborate courtesies, their ritual combats surrounded by pageantry, fanciful heraldry (though they don't get into quarterings), and intense loyalties. Donovan manages to remember a ritual from before he lost his memory, and binds five subordinates to him strongly with it -- so gracious and proper a gesture it was.
Hmm. All of these are science fiction. Certainly I've seen high fantasy worlds enough where men act like free and equal citizens altogether too readily -- and atomized ones, with no connections to each other. . . .
Tolkien had it in The Lord of the Rings, though, so it can't be based on him. From when he first sees Faramir's desperate retreat, Beregond's loyalty is clear -- even if it does, in due course, lead to Denethor thinking him a traitor, for willingness to defend Faramir to the death.
Certainly it was a passion in real life that would be expected in such societies in fiction.
His golden locks Time hath to silver turn'd;
O Time too swift, O swiftness never ceasing!
His youth 'gainst time and age hath ever spurn'd,
But spurn'd in vain; youth waneth by increasing:
Beauty, strength, youth, are flowers but fading seen;
Duty, faith, love, are roots, and ever green.
His helmet now shall make a hive for bees;
And, lovers' sonnets turn'd to holy psalms,
A man-at-arms must now serve on his knees,
And feed on prayers, which are Age his alms:
But though from court to cottage he depart,
His Saint is sure of his unspotted heart.
And when he saddest sits in homely cell,
He'll teach his swains this carol for a song,--
"Blest be the hearts that wish my sovereign well,
Curst be the souls that think her any wrong."
Goddess, allow this aged man his right
To be your beadsman now that was your knight.
George Peele, A Farewell to Arms