I would consider it more of a trade off in many respects.

Demonology certainly has far better AoE, even without Hellfire, so I would think that it would more so fall into the realm of what other ranged classes that you have with you. I am a bit spoiled -- as were the warlocks that ran with my balance druid when we did 10's -- in that I run with a hunter and fire mage, so the adds are never an issue for us.

Primarily, though, I wouldn't really favor demonology for this encounter unless your raid just really lacks in the AoE department because the adds really just are much of an issue until you get to the fourth add, and at that point you should be in a position that you can ignore the add or the Bloods and just push Cho'gall, which should cause them all to despawn.

Dealing with Conversion is a much bigger issue in my opinion because you really want to have the power to stop them as quickly as you possibly can, and being destruction allows for that. I suppose you could kind of get away with Glyphing Howl of Terror, but with the cooldown it would only be up every other cast usually; but you could use Death Coil for the other and rely on teammates to fill in gaps too.

Really I think it just comes down to group composition. There are certainly 10-man comps where it would be more beneficial to be demonology over destruction, but provided your raid has at least one survival hunter, balance druid, or fire mage, I just don't see the need for the additional AoE DPS in comparison to destruction's higher single target damage (although a skilled demonology warlock that can abuse Decimation will do extremely well on single target damage) and Shadowfury.

Another thing that I forgot to mention is that demonology really has the least defensive options for phase 2. Affliction can spec for some pretty decent (not as fantastic as it used to be, but still good) self healing and destruction has Nether Protection which is pretty awesome in that phase.