It's hard not to recommend an affliction warlock for every M+ team right now, at least for pushing higher keys. Perhaps most importantly (and I'm not joking) warlocks can make
Gesundheitssteins for free when
Astralheiltranks routinely cost 200 gold on the auction house. You can easily save a thousand gold per dungeon if you're liberally using
Gesundheitssteins (which you should be when they are free). There is a very noticeable hesitation in warlock-less groups to use a
Astralheiltrank -- I can almost hear the gears turning in my party's head when big damage comes out trying to calculate whether this key is important enough to justify the
Astralheiltrank cost. This has only been an issue since
Astralheiltranks were introduced with Argus because the previous
Uralter Heiltranks were incredibly cheap. It's not an exaggeration when I estimate that running with a warlock has saved me at least a hundred thousand gold.
Warlocks are also key to the “kite meta” that is becoming increasingly popular in high keys. The idea is to tag several packs of slowable mobs, group them up and let the tank establish some aggro on them, and then have the tank run out and start kiting. The mobs will chase the tank instead of trying to use their abilities, and the rest of the group is free to unload on the now-harmless mobs. Warlocks make this much easier with
Sacrolashs Dunkelstoß which causes
Verderbnis to slow mobs by 60%. The warlock just DoTs up the whole pack, and the tank is now able to kite without having to use any movement abilities.
Gesundheitssteins and kiting aside, warlocks are strong in all phases. They do great damage, they are tanky, and they have unreal self-healing even after the recent nerf. Warlocks are competitive in both single target and AoE damage, and their boss damage scales with more mobs so you can cheese some fights by pulling in trash or spawning adds. Warlock's defensive cooldowns let them survive some of the nastiest one-shot mechanics in the game without breaking a sweat (pay attention during your next
Amalgam der Seelen and see if the warlock's health bar moves when
Seelenbersten pops). And if a warlock does take damage, drain soul lets them quickly top themselves off.
Lastly, warlocks have historically been central to a lot of dungeon shenanigans because of their summons and teleports. Summoning allows people to hearth out mid-dungeon and respec then quickly get back to the group. Teleports have been used for several questionable skips, some of which have been hotfixed but many of which are still available.
Warlocks are as close to “necessary” as a class can get right now.
