It seems like I'm one of the few that considers Dori to be terrible, so I thought I'd elaborate on my feeling toward the card.
First, his stats. You'll find cheaper attackers in lore, such as Loyal Hound, Emyn Arnen Ranger, and Quickbeam. In even in a dwarf swarm deck you'd be better off playing Bifur or a cheaper lore dwarf.
Ok, so you're playing him for his ability.
Second, his ability. Let's break it down. If you are using it on a huge attack that is going to kill a full health hero, it's going to kill Dori instead. In which case it's more cost effective to be using a cheap chump ally. If you're using it to siphon off just 2 damage from a hero that is near death (so that Dori doesn't die and can be healed and used again) you'd be better off just having Self Preservation on that defender instead, as it would not only save the hero but actually provide the healing as well (with the Dori strategy you still need another card to perform the healing). If the lack of flexible target is an issue for Self Preservation, look to Daughter of the Nimrodel instead (though I think Warden of Healing is even better even if it only heals 1 damage on each target).
Well, what if the defender can't be healed? Ah, now we get to it. This guy is amazing in a Beorn deck right? Wrong.
Let's break it down again. If you're using this guy to save Beorn from a huge hit, it's going to kill Dori, in which case a 1-cost chump ally would have been better. So that's not why people think he's good. People think he's good because if Beorn gets attacked by an enemy with 2-3 , Dori can siphon off the damage to Beorn. Because while Beorn can't be healed, Dori can - and can then be used again the next round. This strategy keeps Beorn alive for longer.
And it works, to an extent. But you know what's better than Dori for protecting Beorn from 2-3 enemies? Simply defending them with a Defender of Rammas! They're cheaper, they can defend up to 4 (which would kill Dori), you can have multiple of them in play, and they don't require healing every round! And if a surprise shadow attack puts an enemy up to 5 , your Defender of Rammas will die, but so would've Dori. But least you've saved Beorn from taking 4 damage, and the Defender of Rammas is cheap to replace.
There you have it. Even in Dori's ideal use case he gets blown away by a staple ally. Put this card back in your binder.