He was very useful before skill inheritance went online.
Bulky enough to withstand Hector's blow, powerful and fast enough to either put him in a tight spot, or kill him if another unit couldn't.
Also, he was the perfect counter against Takumi.
Then Hector started learning Vantage, and nobody could touch him once he got hurt (Robin not powerful enough to ORKO him).
And with archers disappearing from the Arena, Robin pretty much lost his meaning.
He wasn't always bad. Besides, Armored Grima and Xmas Robin are both plenty powerful (though Xmas Robin requires quite a bit of investment).
Nowadays, I run Thunder Reinhardt and Brave Lyn (with Brave Bow). They're a killer-combo, especially with Azura behind them.