Spock needs someone to keep his green-blooded computerized ass in line, even in his older, wiser years. Kirk had always provided that role, balancing humans values such as compassion and forgiveness against Spock's logical deductions in all their missions together. Just look at how out of control things almost got when Spock tried to go it alone during Unifications. To quote Sagan, Kirk had a "Baloney Detector" that sure would have come in handy in Spock's dealings as an ambassador. Spock had a tendency to come to a conclusion that seemed logical, and dismiss other avenues that might hold merit, but not logic.
Spock already showed a lack of objection in his opinion of Valeris. Of course, who could have guessed that a fellow Vulcan would have behaved so illogically.
There was also the larger symbolic role of Kirk, someone who had so many run-ins with the Klingons over the years that if he could overcome those differences and work with them, than anyone could. He had stuck out his neck for Earth so many times, and stuck with his values through thick and thin and at the detriment of his own career that Federation citizens couldn't help but trust him. It would show that they only had one sensible avenue to pursue here...cooperation.
As Spock so succinctly put it, "Only Nixon could go to China." Damn, this film had good writers.