To my mind, it wasn't so much about galactic socio-political aims. It wasn't about Romulan empire-building or conquest. It wasn't about cultural ramifications of Vulcan influence on Romulan society.
It was about Spock. It was about showing Spock one last time, giving him a send-off legacy of galactic proportions, something to rival Surak himself in the annals of Vulcan history.
Kirk was given his heroic send-off role in TUC, the man who brought peace between the Federation and the Klingons, a noble legacy.
What heroic achievement would Spock have, something to rival Surak's importance? Being the Vulcan that unites the two divided peoples (Vulcans and Romulans), the man who brought peace between the Federation and the Romulans.
That's pretty much it. Some momentous event of galactic importance where Spock plays a pivotal role, to secure his heroic place in history one last time.
I think basically that's why Nimoy agreed to do it. He felt that what he did actually was a contribution to his character as well as the fans of Spock.
He would probably not have agreed to make a McCoy-like appearance.
An interesting thought is if TNG writers ever had the slightest thought of anyone else beyond those of the TOS cast who actually appeared, no matter how small or in what form the appearances would have been. Did they ever consider Kirk, Uhura, Sulu, Chekov or Janice Rand?
Or any one-time ally or villain in TOS? Khan? One of Kirk's doctor girlfriends? Any other Vulcan?