Yeah, Deathwing might be a crazed dragon, but I don't think he's stupid. The less dragons to fight him, the better. The red and green dragons both have their aspects, and the bronze have the majority of their drakes left (I don't know if we ever found out where thier aspect was)

Instead of letting the blues (Who obviously have a bone to pick with him) live without an aspect, it would make more sense to try to wipe them out completely, to prevent them from assisting the others. Sure, without malygos they aren't as strong as the red dragonflight, but even a weakened group of drakes are capable allies. Not to mention if somehow Kalecgos manages to reach aspect status by some means, and he's anywhere near as strong as Malygos was, or at least smart enough and influential enough to rally the blue dragons that are left, then deathwing is going to run into some problems.

That being said, I'm wondering if we might see a reemergence of the Netherwing in this expansion. I'd love to see one of the aspects try to rally the nether drakes to fight against deathwing, it's unlikely as it doesn't make much sense, but it seems like it'd be a good time to introduce them to azeroth.