Sure, he can defend against two trolls -and then attack even. That's one of the main uses of him, I think (though we have rarely ever managed to have him as an ally).
Our only play so far of Carrock (yes, way behind on my LotR plays), Gloin was sitting pretty on about 7 unused resources when Grimbeorn popped up. He hadn't gotten the Song of Wisdom on him yet and most of the cards in hand were Lore (in a Lore/Leadership deck), so they were just wasting away. So thanks to the one location that allows spending any resources as Leadership resource, we paid Grimmy off in a single turn.