Since Necrosis procs off Rune Strike there is no reason to not take it, especially over talents like Virulence and Corpse Explosion. I do remember that Necrosis was accountable for around 8% of my total damage done on most non-AoE bossfights
Also Vicious Strike is quite pointless since your main FU ability is Obliterate which isn't affected by it. Not to mention Outbreak which is very lackluster until 3.1 and the buff to Plague Strike.

PS: BcB IS a bad tanking talent since the additional strike can be parried, but I suppose it too would procc off RS (never tested it though and too lazy to read it up now)

PPS: This is how I personally would switch around the points in the unholy tree

edit: nevermind

In terms of single-target tanking as a 2H/frost tank, what is a better use of runes: Oblit or HB? I find that my HB crits for much more than my Oblit and it costs less runes. So why would any tank use Oblit > HB? Other than single-target tanking I can't think of any situation where a tank would want to use an ability that does less damage and costs more.

I rarely use Oblit. Like Verik said, it seems useless to use it when it costs more and does less damage then a lot of my other abilities. If it's up and I happen to see it, I'll use it, otherwise it's not in my main rotation.

Even on single targets HB crits way more often and does more damage all around for me. I would stick with HB.

HB and Obliterate both cost 1 Unholy and 1 Frost. Also, HB goes on CD when its used so you can't spam it. But generally I use HB whenever Killing Machine procs and I don't have enough runic power for Frost Strike.

I use obliterate, because it does more damage when Glyphed than HB, it scales better with better weapons, HB don't. I can spam it in my rotation (which uses about 4-6 Obliterates in 20sec period depending on Death Runes).
Oh, and i use HB in AoE tanking or when Rime procs, and only then.

Wow don't I feel stupid...I can't believe I thought HB was only 1 Frost....


Anyway, thanks for your answers but I'm not sure I really got a definitive answer. I guess what I'm saying is that if I were tanking a boss, with no adds, (assuming I am not spec'd for Death Runes) is it better to just use Oblit? I think I already know the answer, I just want to make sure that I should be incorporating it into my main tanking rotation.

I have a question, why exactly is Betrayer considered BiS for Dk tanks? I wasn't able to find any definitive data on it.
It looks like Inevitable Defeat would be the better weapon.
Am I missing something?
Sorry if I'm not posting this in the right forum. My guild was telling me that ID was BiS, and I have seen that Betrayer was - anyone know the math behind them?

This is a fun question and, imo, it falls down to racial +exp for these three weapons.

ID/Death's Bite/Betrayer.

ID will be used by most due to the +EXP. This will increase TPS more so once you are hit/exp capped.
DB adds a substantial amount of +HIT while not having any wasted stats.
Betrayer has top DPS but lacks +EXP or +HIT, also, the lack of +STR drops our Parry slightly.

It breaks down to what you want to use and your style of tanking. If you're HIT/EXP capped, Betrayer's DPS will increase your TPS. But, if you're like a few of us and push EXP, the ID is going to benefit you more so.

(My knowledge of ID refers back to Notebook's posts as it's only dropped once for my raid and it went to a Fury Warrior.)

Personally, I use DB since that's all that has dropped and letting the Betrayer go to DPS is priority.

Everything you posted there boils down to TPS. ID is definitely the BiS dk weapon if what you are looking for is survivability. It is the only high stam/agility weapon.

great, thank you guys for your responses -much appreciated.