Always the same ****ing bull****. Don't comment if you don't know what you're talking about, and you clearly don't.
@op:
Affliction is viable, and it's a great idea to have 1 per 25 man raid. They bring 3% more damage on their CoE, 5% reduces physical damage from the boss (often overlooked but really nice), have the only long range instant cast spammable snare in the game (Curse of Exhaustion, great for fights like Vashj!) AND because they don't sac their pet, they can provide the tank with Blood Pact (the Imp stamina buff). There are a lot of benefits to the raid for bringing one Affliction Warlock, but bringing so many buffs over other Warlock specs means you'll do less DPS than those other specs.
You can also look into Demonology, which does more DPS than Destruction until high T5-low T6 but is a lot harder to play (due to pet management, pet dies = DPS gone).
And even as Destruction there's outplay potential. The garden variety noob Warlock will only cast Shadowbolts while good Warlocks know that Immolate, for example, is a DPS increase and that you should spec into 5/5 Imp Immolate because of that, and keep Immolate up when you are not Bloodlusted. Of course only if there's a Fire Mage using Imp Scorch.
Just keep a spreadsheet with your numbers so you can compare the specs, which spells to use etc etc. You can easily squeeze out an additional 100-200 DPS by changing your spec and optimizing your rotation.