In one of my truly insane moments of inspiration, I just figured out how to use Spock as more than just a throwaway NPC to be rescued.
During the Spock Rescue mission, the PCs get jumped by a couple of dozen Borg drones and a scout ship from the attacking Borg force. Their target is Spock who they want to make into their new spokesman, ala Locutus.
Spock is of course an even better choice that Picard. He's a respected ex-Starfleet officer who probably served under Starfleet's most capable tactician, he is the Federation's leading ambassador and probably knows more secrets than any other single Federation officer.
If the Borg assimilate Spock it will go a low way towards defeating a war weary Federation. Spock knows all the secrets and he probably knows Starfleet, its tactics and strategies and he knows how the Federation will react to the crisis and take appropriate precautions.
Arise, arise, Riders of Theoden!
Fell deed awake: fire and slaughter!
Spear shall be shaken, shields be splintered,
a sword-day, a red-day, ere the sun rises!
Ride now, ride now! Ride to Gondor!
Theoden King: The Return of the King