Tenacity is better than dh as is. You gain 80% of the dos benefit while taking less damage. Less damage means more healer dps. It's overall a greater dps gain than dh.

As for melding, the reason it seems useless is because melding for anything is useless. Dh gives pitiful amounts from melds as well.

Ten scales no worse than dh. It might scale much better depending on how the damage reduction works.