Obliterate doesn't do more with a third disease, its just more damage in a build that gets damage from more sources than just about anything. 2/51/18 isnt far behind 18/54, its only about 200-300 dps in the sim, thats pennies when you take it to a raid. However, when you consider that its going to lose 10% of its damage when the pet dies 2/51/18 looks even better. I think if the sim randomly killed off your perma ghoul we would see more realistic numbers and 2/51/18 pulling ahead of it. Then you have to consider that 2/51/18 brings a rarer raid buff than 13% spell damage, 2/51/18 looks even prettier. 2/51/18 has better AOE now that UB is single target. I think they are just about equal but i rather have a commando build ready to handle any situation and brings it all to the table than the 18/54 that specializes in single targets. Ill most likely take vacations from frost to visit unholy though.
1. "Obliterate doesn't do more with a third disease" - What?
2. "2/51/18 isnt far behind 18/54, its only about 200-300 dps" - Are you talking about 17/54 or 18/53, as these specs play very differently? Either way, 200-300 dps in a sim is 200-300 dps in a raid, i would hardly consider that pennies.
3. "I think if the sim randomly killed off your perma ghoul we would see more realistic numbers" - Would you also like to add breaks for running, aoe and bursty moments? This is simming, you simulate the optimal raidsetting, a Patchwerk; there is no raid damage, there is no aoe, there is nothing but straight on single target dps. And even if you would take raid damage into account the only encounters in Ulduar where a perma ghoul-DK would lose his pet would be Mimiron and Yogg (although you just summon it for brain room anyway). Point moot.
4. "Then you have to consider that 2/51/18 brings a rarer raid buff than 13% spell damage" - Rather assume that all needed raid buffs are brought already, and that if you go with unholy a shaman will be there to carry the haste-buff. We simulate optimal DPS for ourselves, not for a non-perfect raid.
5. "... than the 18/54 that specializes in single targets." - I'll have to assume you are talking about the 17/54 spec then, as the 18/53 still keeps WP and thus 4/5 of it's AoE-arsenal, which if you ask me is just enough. It feels like 18/53 will be the new competetive UH-DW-spec anyway as it will scale better, with Obliterate, than 17/54 (and as mentioned it keeps its AoE).