Robin may only be good for Takumi (and a lesser extent Kagero), but the Facepuncher is so common it helps to have a hard counter to him. Plus he has Spur Defense, so that helps for tanking. And I gave my Robin Rally Defense (so something could buff Nino), so even more tanking.
I am considering replacing Robin with Eirika though so I can go all Nino Kuni on everyone. Problem is my Eirika is -Attack, and Lucina is so much better at being a Red Sword, and I'm very hesitant to not use Lucina right now. Leaning towards replacing Robin with Sharena eventually, but I'll see how that goes. Nino is probably my MVP now even without any buffs to help her.