Yesterday, a player posted on the official WoW forum asking about the current state of the Doomguard and Infernal Warlock pets. It's a hassle to learn how to summon these demons, requiring the completion of multiple quests for each, and summoning them requires reagents, including the death of one of your party members (for the Doomguard at least). With all these requirements, one would think the pets would be worthwhile, but their cost and unpredictability (they have to be continually enslaved, which is on diminishing returns and may break at any time) make them worse than the normal Warlock pets.
Eyonix answers the original poster, saying that the devs are concerned about these pets and will buff them sometime in the future. That "future" may be the next expansion, but this is a better answer than the class has received on the subject in a long time. To the best of my knowledge, previous answers were something along the line of "These spells may have situational uses and will not be changed from that role". Now there is once again hope that these cool and iconic (watch the original WoW cinematic, if you don't recall) Warlock pets will become more than simple novelties.