Also, why are so many people ignoring silver arrows and silversheen?
Because, ranged attack bonus on a non-dedicated ranged character is a generally a joke. 1d8-1 silver arrow damage isnt going to matter if they cant get past your +4 natural armor.
And silversheen is an obscure alchemical item. I can count the number of times i have seen NPC's have it on a beholder hand.

I think you're overestimating the power of DR 15 versus a barbarian, and underestimating the downside of -6 Intelligence and Constitution --.

(1) The damage for a raging, Power Attacking barbarian is around 2d6+13 to 2d6+15 at level 3 (the lowest ECL you think the huecava should be at). So right from the start, barbarians will damage you on most or all hits. At level 6, the damage for a raging, Power Attacking barbarian is around 2d6+21 to 2d6+22. Obviously, your DR is still helping a lot at this point, but it's likewise pretty obvious that it's going to get outclassed by barbarian damage pretty soon, and we're not even into the teens yet. And with a morale pool that's probably a third behind a typical cleric*, and no additional defenses against magical attacks, that DR is just enough to get by (when it comes to barbarians, that is. Gnome sword-and-board fighters, you can kill all day--but then so can a crusader).
So.. DR 15 allows you to survive a hit from one of the most extreme damage outliers, and you downplay it?
Yes raging barbarians will damage you on all hits. They will also kill most others in the same hits.

So as such, that isnt even relevant. In the wast majority of relevant cases your not going to fight other PC's.
How many times have anyone fought an encounter at level 1-2 against someone doing 2d6+22 damage?
The high end of damage i can find around level 3 is a Ogre smacking you for 2d8+7. Thats only going to tickle most of the time.