The extra toughness is pretty handy in melee. Orks are used to going last anyway.
Most of the dark eldar's poison is in guns, orks are used to guns wounding them on 4s and allowing no save. Of course then there's wracks.
I've seen lots of people building Dark Eldar, but I haven't seen that many people playing them. In fact I'm pretty much the only one i've seen use them.
Uge Choppas on nob's mobs not popular I'm guessing?
For shooting its pretty much grotzookas and then their subpar battlecannons.
Killa kans would be good against dark eldar, but you'd need a lot of them to survive the lances. You're also hitting on 5s in melee against some things.
Tau aren't that bad for S6 guns.
I've ordered a venerable dread to convert into a librarian, but the speed at which my dreadnoughts die and the reliability at which my librarians die makes me doubt them.
I was planning on being very careful with him and flying him from cover to cover until the rest of the army was in position for a combined strike.