The adds are secondary in nature on the Halfus encounter. Killing them reduces the healing strain, but it doesn't provide you any tangible benefits beyond that; which is why you never kill the third drake.

Dragon's Vengeance is applied as soon as you release a dragon, not when it dies, therefore Halfus will take increased damage the moment that you release one. I do not know if Bane of Havoc dips from Dragon's Vengeance if it is placed on Halfus, and I coudl not find anywhere which mentioned the mechanic nature of the spell to theorize if it would. This is why I asked for clarification on the matter.

However -- if BoH doesn't benefit from DV when placed on Halfus, then there is really no reason at all to put it on him and then DPS the add. In this situation, you could either deal 150/200/250% damage to Halfus, and 15% of that to a dragon, or you could deal 100% damage to a dragon and 15% of that to Halfus.

In essence, say you have two dragon's released, causing Halfus to take 200% damage. Your Conflag would normally hit for 100,000 (because I like round, easy numbers.) Hitting Halfus with Conflag would deal 200,000 damage and then 30,000 to the dragon; for a total of 230,000 damage done.

If BoH -does- benefit from DV when placed on Halfus, then, yes, by all means DPS the dragon (the damage would be the same either way.)

Whichever you choose is rather irrelevant though. So long as two dragon's die before 50%, then your raid is not in any danger whatsoever. The only reason that burning an add down faster might matter would be to get the third dragon released for their debuff on the encounter; which generally won't make much of a difference regardless.

Your third dragon is almost always going to be either Slate or Nether (the only time it won't be either of those two is when you have Time, Whelps, and Storm, in which case the third would be Whelps.) Slate is a fairly insignificant reduction in incoming tank damage; not to mention unpredictable. While beneficial, not having it out for a few seconds longer isn't going to cause a wipe -- and if it does, then you have mechanical/gear issues which you switching off of Halfus will not solve.

The dragons themselves will fall rather quickly; more so if you use Bloodlust to kill them. Whether or not BoH benefits from DV is rather moot; there's still little actually gained from DPSing the dragon directly when you can deal significant enough damage to it via BoH. And if BoH doesn't benefit from DV, then there is no reason not to DPS Halfus.

The only set up in which I would agree that being on the dragons as Destruction is worth the gain would be on Time/Slate/Nether or Storm/Slate/Nether and that's only because the Slate/Nether combination is the only one in which the Halfus tank should be taking significant enough damage to warrant it being an issue.