On release I like this card, viewing it as one more piece of the developing Ent deck. Since then my appreciation has grown, and the Derndingle Warrior is pretty much my go-to defensive ally in most contexts now as well as one of the key components of the Ent deck.
So, the standard Ent disadvantage of 'Enters play exhausted' is a problem, but once that's resolved, what do you get? Well, for starters there are very few Sentinel allies costing less than 3, which can be significant for multiplayer. Of those, the Derndingle Warrior's 2 /3 makes him the sturdiest alongside Dwalin (who is unique, and only cheap in a Dwarf deck). Of course they're still not the best defensive stats for the long term, and options for boosting them are limited by the Derndingle Warrior's inability to have attachments (though a Hauberk of Mail would go nicely), but then I haven't mentioned the ability yet.
Now the Derndingle Warrior's ability involves dealing damage to him, so if you want to rely on it repeatedly then you will need healing and/or hit point boosts, but in exchange for that 1 damage the Warrior boosts up to 5 , which is the best you're likely to see on an ally and enough to not take damage from the attacks of the vast majority of enemies in this game. If you can get past that one round where the Warrior is exhausted and give him a bit of support he can easily cover all defences for you for an entire game.
As noted, he is limited in potential boosts, but there are options - in addition to the already-mentioned Hauberk of Mail, he's obviously a great target for an Ent Draught since his ability gives you all the you need so long as you have the hit points to keep triggering it. And of course Narya is a great one with Ents in general - note however that the Derndingle Warrior's ability only lasts through one attack, so if you have him defend multiple times you may need to deal additional damage (though not necessarily, since after using Narya he's already at a base of 3, which may be good enough without the additional boost. Whether you use these options or not, the other key element for supporting a Derndingle Warrior is some form of healing - with the obvious options being Warden of Healing, Imladris Caregiver or of course Wellinghall Preserver (and even in a dedicated Ent deck which wants to keep its Booming Ents at high healing them works since you damage them again when they defend and therefore before you attack).
The Derndingle Warrior is obviously a big deal to an Ent deck as their strongest defensive option, but he can also fill that role for any number of other decks which otherwise have nothing to do with Ents. He's just that good.