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.