It's also worth mentioning that Bane of Havoc currently double-dips from the 8% spell damage modifier, so make sure you put curse of elements on both targets.

He advocates it for the healers. So your DoTs don't explode on everyone near you.
It is a DPS loss, but only for a few seconds of applying the DoTs on the other dragon

@ Twill
He is advocating unglyphing Soul Swap so that Soul Swap removes all your DoTs from the target. The intention is, as he says, to stop your DoTs from ticking and keep you from exploding and doing damage to those around you.

However, removing the glyph is a good deal more than a few seconds of DPS. The glyph makes it so that Soul Swap does not remove your DoTs from your target, but instead "copies" them. By Soul Swapping from one dragon to another, you spend two global cooldowns, but apply three DoTs, including Unstable Affliction, which has a cast time. You can use Soul Swap in this way throughout the entire fight, as both dragons will be in range, and almost double your damage. Unglyphing Soul Swap is HUGE DPS loss.

In addition, as later commenter noted, if your raid needs you to unglyph this to avoid wiping, you are doing something wrong.

One strategy many groups have adopted that avoids the problem of Engulfing Magic is to have melee stack on Theralion, while ranged and healers follow in a semi-circle, all ten yards apart from each other. They must be vigilant and move into the melee group when they are targeted with Twilight Meteorite to share the damage and avoid dying, but it removes Engulfing Magic as a threat entirely. As warlocks aren't the only class that has problems with this mechanic (and some, like Resto druids, have it much worse) this would certainly be my preferred strategy, which has the added benefit of allowing you to DoT up everything in sight :)

stupid question perhaps, but how do you click through to see spells used etc?

Are you really trying to say that limiting your dps is a GOOD idea? The whole point to pushing dps is to actually complete hard modes properly. Advocating dropping dps because you're not smart enough to move when you have a debuff means you shouldn't be trying to push current content. Also, don't make excuses saying "It'll make the healers lives easier" blah blah blah. How to make a healers life easy on double dragon: Get the hell out of the raid when you have the caster debuff exploding on everyone.

@Tyler
Forgoing one of your most effective tools because you are doing "enough" DPS is simply nonsensical. The use of Soul Swap on this fight is an amazing mechanic only available to warlocks and while I think no one will disagree with you that blowing up the raid is bad, the situation simply does not warrant passing up this opportunity. Would it be bad if we had the terrible misfortune to be targeted with Engulfic Magic and Twilight Meteorite? Sure. But Resto Druids and Shadow Priests would be in the same situation, and the raid would deal with it. It's not THAT bad.
I'd also say that if your raid really IS wiping to you getting targeted to both of these things at once, I mean, I'd have something to say to your healers, but it probably would be better to just spec Destruction. A lot less DoT damage, great single target, Bane of Havoc is awesome, and really less of a headache all around.
(Target switching the two dragons every soul swap cooldown is unbelievably annoying.)

I feel like you are speaking very logically, but you haven't actually played as a warlock on any of these encounters in a real life raid situation. Later in the comments you argue about the use of AoE and DPS prioritization on Halfus, and you are just wrong. Are the dragons secondary targets? Yes. Does Destruction have shitty AoE? Yes. But if you think that you would ever be ALLOWED to ignore them based on either of those criteria, you have just never done this fight with your tank calling out interrupts and your healers screaming for mana and your raid leader yelling at you wondering why the dragon isnt dead YET. If you know your raid can handle it for you to stay on Halfus full time, go for it, but this early in the expansion we have to put the DPS where it is needed even when it isn't ideal for us. And I have yet to kill Halfus when my Raid Leader was happy with the speed at which the drakes died. Deal with it. Sure you are going to less DPS because your AoE sucks but it is better for the raid if the adds die faster and you have to do it.

In addition, while Bane can be an amazing tool here, this is another situation where I think Affliction is probably the better spec due to Soul Swap. It is a lot easier to justify to your raid leader the loss of a few GCDs on a slow dying dragon for big return on Halfus, and man, Soul Burn Seed of Corruption + Jinx is great.

@Unfixed
I'm not an expert at World of Logs, its a really really confusing place but when you can make it do what you want it to, it is great.
There are two ways you can click through to see Soul Swap casts. I am using the first US parse to avoid language barriers.
You can Analyze/Damage Done/[Warlocks name]/Damage by Spell
http://worldoflogs.com/reports/7ayna5n5clmom8nl/details/31/?s=0&e=500
which shows you the total amount of spell damage done by the warlock, the number of times they cast certain spells and how much they hit/ticked for.
OR you can Analyze/Damage Done/Damage Done + By Spell + Source = [Warlocks name] + Target [Pick a Dragon]
http://www.worldoflogs.com/reports/7ayna5n5clmom8nl/analyze/dd/spell/?source=31&s=0&e=500&target=18
This shows a graph of the fight and shows you how much damage each spell was doing at a particular time. It is really easy to see where Soul Swap was used, it isn't always this spell.

Also, disclaimer: there are probably better/more efficient/less confusing ways of seeing the information you want, but that was how I did it.

I'm really not understanding why you find it so important to cut your damage for the debuff. It LITERALLY takes a maximum time of 2 seconds to (first) react and then get max distance needed to not damage people. We have a resto druid in our raid that keep's most of the raid HOTed at all times and we rarely (if ever) have an issue with people taking any serious damage from it, let alone be killed. The only reason this debuff could be a slight problem is if:
A. You have ping over 600ms
B. You have a brain with a ping higher than 5s

So, if you think you're being very competent by stopping all your damage for 5-10 seconds, you're actually making it worse on healers mana because you keep extending the fight. Suggesting all dps to stop for this trivial debuff is rather idiotic.

If the drake being alive did not affect healer mana, we would simply release them and not bother killing them at all. You seem to be contending that at their worst, the dragons merely need to die before Halfus hits 50% and begins really driving us crazy with stuns. If the dragons were this unimportant, then everyone would do what you are advocating warlocks should do: essentially let the dragon die to splash damage. I am just not seeing how this is possible. Killing the dragons is the hump to get over. Especially when you have something like Malevolent Strikes that require your tanks to taunt back and forth, having a second dragon to deal with is extremely trying for tanks and healers both. And, while healer mana can be used at any point during the fight and getting Halfus to die quicker is in everyone's best interest, mana does not deplete in a linear fashion. The fact of the matter is that more damage is going out when there are more drakes alive. They will be using more mana per second at that point than at any other time, and since they can only cast so many heals per second, you want to reduce the time spent in a phase where RNG will wipe you. Once the dragons are dead, mana recovery is much easier and there is a good deal less raid damage.

And finally, I will concede that if you are having that much trouble dying to Engulfing Magic, don't use Glyph of Soul Swap. I think if you can't use the glyph, you shouldn't bother playing Affliction because Destruction gives yields higher single target damage anyway and is much easier to maintain during movement heavy fights. I will say however, that I am shocked you have so many problems with Engulfing Magic. In early weeks, our Resto Druid was often slow to move and did a good deal of damage to the raid before he got out of range, but he never wiped us, even in Tree Form. We eventually adjusted our strategy, which made the entire phase much less prone to spike damage -- provided everyone was paying attention to the meteor. I think what I am saying is that removing the Glyph of Soul Swap is really the least ideal way to deal with the mechanic, and you lose a lot more than you gain.