He's talking about the raid DPS rotation.
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You're wrong in a couple of ways.
1. DS/Ruin is the best consistent DPS raid spec in Vanilla for Warlocks, it's been number crunched to death. It has terrible utility though and suffers in PvP, so most people run SM/Ruin because the difference is small only if the Warlock is allowed to use Corruption. You'd have probably at most 3 Warlocks actually allowed to use Corruption in a raid (that's actually too many but you did see it), and before the cap was moved from 8 to 16 debuffs you'd likely have none.
2. Warlocks put up their curse, used Corruption if they were assigned a debuff slot, and then spammed SB. That's it. Anything else in raids was just poor debuff management. No Curse of Agony or Doom, no Siphon Life, no Immolate. And if you were running SM/Ruin but didn't have a Corruption slot assigned to you then you really ought to have been raiding as DS/Ruin.
Assuming you're not talking about 'rotations' while leveling or something dumb like that because rotations are only relevant to dungeon and raid content. DoTs scale terribly in Vanilla and you're always better off getting an extra SB off than applying anything but Corruption, which is really only worth applying because it gives you a chance to get more SBs off.