Utility is perhaps at the forefront of the balancing game right now. When it comes to which specs a raid wants to bring along with then, that added bonuses those specs can bring to the table are often considered first. Damage has been pushed aside. Everyone has damage, but not everyone has the same utility. To many people, utility is just those over-homogenized buffs or debuffs that they bring along with them. While a part of the equation, buffs and debuffs are really only a minor part of utility. Just like damage, they've been given out to everybody and anybody to the point that none of them hold any special weight anymore. Instead, utility can be defined as anything beyond a give role -- tanking, DPS, or healing -- that a player can use to benefit the raid or themselves in some way.
In this instance, the ability to heal yourself for large amounts of damage via talents and Drain Life would be considered utility. Even though Drain Life as a filler spell for affliction has been stomped on by Blizzard, the damage that it can deal isn't so abysmally low that it's never worth using. A player's ability for self-healing can go a long way in helping to down encounters, especially in terms of world firsts. Less reliance on healers generally means that fewer healers need to be taken, the fewer healers that a raid brings along, normally the harder the content they can push. The world-first Rag kill had nothing to do with healing and tanking and everything to do with damage.
That is affliction's unique utility: its higher rate of self healing, as well as its ease of picking up Jinx, which is the best AOE method of applying the spell damage debuff. Even though self-healing is an exceptionally strong part of utility, the problem that we predominantly see with affliction is that it just isn't enough. Passively, affliction doesn't generate much more, if any, health than the other warlock specs. Self-healing is a part of the warlock theme overall, and every spec has some method of either increasing the healing it takes or healing itself through damage. Affliction's largest benefit is that it can use Drain Life with the least loss of damage.
Giving up damage to gain utility
The give-and-take style of utility has never worked in the past, yet it is continually a concept that Blizzard has clung to. Players do not like to give things up. Asking a player to lower their damage in order to bring some form of raid based utility just doesn't work inside of a system where that isn't an acceptable norm.
A vast majority of classes do not have this trade-off, or if they do, it isn't perceived in the same light. Take druids as an example. As a balance druid, I'm often asked to battle rez people or use Tranquility. Both of these require cast times and two GCDs, one for the spell itself, another for shifting back into Moonkin Form. Ask any druid if they would think twice about using those abilities in a raid and they'd laugh as though you asked if they were from Mars. Of course they'd use them. Now, go ask a warlock how often they use Drain Life inside of a raid.
Overall, the damage disparity is probably rather similar. A warlock that uses Drain Life for the sixth and seventh Flame Scythes on Staghelm probably wouldn't lose more DPS than a balance druid asked to use Tranquility and Rebirth on the same encounter, yet you would never see the warlock using Drain Life. Why? Because players don't accept the loss of DPS for personal utility gain.
Tranquility and Rebirth effect the raid in a noticeable way. Everyone in the raid is being healing, a person is being brought back to life. These things, you see. A warlock who uses Drain Life only seemingly helps themselves. The truth is that many group healing spells that players use are smart heals that target players with the lowest percentage of health, so by healing yourself, you no longer become a target for these heals. In effect, you are saving healer mana and time, which can save lives in the end.
The problem is that Blizzard needs to realize that players don't see the game in the same way that it does. Blizzard has to deal in absolutes, in carefully crafted simulations of what could be. Players, for the most part, focus exclusively on how they can increase their damage. Save for those few that are pushing the top of the top, the notion of hidden utility just doesn't enter into the equation.
Should we want affliction's utility to be focused on self-healing, then Blizzard needs to find a way to make it less give and take. This game has moved past the era where utility can be brought at the sacrifice of damage; instead, it all comes down to those who can give the most while giving the most. With this mindset, we can't have the added bonus for bringing an affliction warlock be tied to lowering our damage.