mech101v
a lock of anyspec does great dps ...
Actually, it does depend on the situation. A lot of people think warlocks are easy to play and are mindless killing machines. A warlock can be easy, but to truly play it well and understand it well is not as easy. Contrary to what some people think, warlocks actually have a lot more going on than fear, shadow bolt, DoT.
As Karnokoto said, a destro warlock will often be higher DPS in raids, while affliction will often be higher damage in PvP. But that doesn't mean that all raiding warlocks have to be destro, while all PvP warlocks have to be affliction.
I raid with an affliction warlock. I occasionally do battlegrounds with her too. When I raid, I am often some of the lowest DPS (if the group is good that is); when I PvP, I am often top damage.
Affliction warlocks are great in battlegrounds because we are all about the fast DoTs that will kill someone even if they ran away. Destro warlocks are great DPS in raids because they often have high crit. Many destro warlocks also go up the demo tree just enough to get the talent to sacrifice their pet for 15% more damage. Affliction warlocks tend to have lower crit since their DoTs don't crit anyway. Also, lots of trash dies before an affliction warlock's DoT can fully run their course, so that was untapped damage right there.
As an affliction warlock, I can get treated as utility. For most fights I have done, I take my imp out and keep it out for the stamina buff while other warlocks may sacrifice a succubus for more damage. When a warlock had to curse of tongues the face boss in Black Temple and pull out felpuppies, I got chosen because the destro warlocks wanted to kill him fast with their big crits. So while the destro lock next to me was doing 1700 DPS, I was doing CoT and summon felpuppy.
So depending on the fight, there actually may be a lot of odd jobs that I have to do. As much as I would kind of like to sit back and mindlessly cast shadow bolt sometimes, one thing that makes warlocks so interesting is all of their abilities. There is no one correct spec for warlocks because everything warlocks can do comes in handy for something. If your guild thinks there is only one correct spec, it could be due to a lack of knowledge of the variety of raiding situations, or they could be saying that because that is what they need for raids at the moment. Either way, don't let people scare you away from a class by saying there is only one right way to spec, gear, or play. It's your money, and you should have fun playing your character above all else.