The Macross is/was the biggest symbol of the UNS in existence, The ship that helped save the human race. using it would be a huge morale booster. I mean, think about it. Reactivating the SDF-1 for a fight, would be almost akin to a countries Prime minister/president/other high office holder, going to the battlefield and personally punching the enemy in the face every time it won a battle.
Or there's the whole.. Along with the 1st Galactic idol, the ship united Zentradi and Humanity. What better Propaganda tool to use to reunite fractured human ties