I would normally agree with that statement, except now I do believe they are VERY USEFUL.
Last night, RottedFlesh saved my life quite a lot. I had 1-3 Melee classes and a hunter chasing me midfield when I had the flag. Rottedflesh came by, DoT spammed everyone really quickly, then feared everyone off of me.
That game we had on healers, we didnt win...but He was not "useless".
I'm not saying either that a Shadow Priest is not better than a warlock. a Shadow priest is a LOT harder to kill than a warlock. Priests have 15% reduced damage taken AND they can bubble, heal, dispell, silence, etc.
But he held his own pretty well considering we had no heals that game. He had an imp and I was running around with over 6,000 health lol. All He did was fear spam people off of me and DoT spammed everyone...
Definitely not the most "fun" class to play, but I will say that they are not completely useless. If you know how to DoT spam everyone, and fear people off of your Flag Carrier, then you sir are good in my books.
Honestly, Warlocks aren't that bad of an idea to have on D. You can have a Hunter of course on D, a priest or paladin healer, then you have your "support class". Obviously some support classes are:
Enhancement Shamans
Shadow Priests
Balance Druids
Warlocks
(probably didnt list them all)
Each support class brings something that "supports" the their team. Shadow Priests without a doubt gives better DoT damage than a warlock and on top of that they can cast their bubbles on their teammates, single-target silences for several seconds, and AoE Fearing with a semi-long CD. On top of all that, they have good survival.
However now you have to look at what Warlocks brings to the table. Depending on their pet, they can keep 1 target disorientated for several seconds, give their entire team 32 Stam, silence a specific player when they cast, or allows them to give them extra survival with sacrifice. They have "okay" DoT damage. They have good single-target fearing that lasts for several seconds. They can also make health stones to pass around to their teammates.
In some ways they are not as good as a shadow priest, but after seeing a good warlock play last night; it's hard to say that they are "useless" when they alone saved my life countless times.