Tbf I've seen maybe two OS advies switch realms like that and one was justified by other IC reasons.
Of the 6 players who have both a noble and an advy in OS, 5 brought their advy from whatever realm it spawned in. That's half of OS' advies. That sixth guy might have done the same thing had he been less lucky and spawned elsewhere. Of the four other advies, one is my own, that randomly spawned there. Of the three others, another moved there from elsewhere. The two last were created there and have no noble in that realm.
i.e. 60% of OS' advies were moved there from other realms, and 60% of OS' advies also play a noble there. The situation is the same in Angmar.
The only allegation I'm doing, is that "these advies are completely free-willed characters acting independently" is utter bull. The majority of advies are created with an OOC purpose to serve pre-determined factions.
