Its a cost / benefit decision. Tossing DoTs on something takes what? 2-3 GCDs (Cor, UA, CoA for an off target?) Its really a question of what else your going to do with that time. As a lock there is a diminishing returns on your single target DPS. After your dots are up how much dmg does Drain Life really accomplish?
Is there any reason not to spend the globals to dot up the DK and make him a viable switch target. It also forces the paladin to choose who to heal, and a heal on the DK is a heal *not* on the war which is a round about way of putting more pressure on the war.
The games that we have last like 30 seconds vs. TSG, sometimes even faster than that. My main priority is putting out pressure on the warrior, so haunt/ua/corr/coa, and whatever nightfall procs I get. I then get to a position where I can fear the paladin twice because the first one is trinketed usually. As I'm moving towards the paladin I roll a couple dots on the dk, but the games are usually too fast for this to amount to anything.
Basically the game goes like this:
Warrior charges in, I roll full dots on him, hunter scatter traps pally almost instantly before sacs if possible and then puts out as much damage into the warrior as possible. I then try to fear the paladin as he will have trinketed and sacced after the scatter trap, and then he has to bubble. At this point I refresh dots on the warrior if necessary, and if the paladin is still bubbled, I tab some dots onto the dk with lack of something better to do. DK is only switched onto if the warrior walls, and as soon as wall is down, we just kill the warrior. Once the pally has trinketed and bubbled (usually 20 seconds in), and your fear comes off dr, game ends pmuch.
This is all assuming that the tsg trains me and not my paladin or hunter. The game is a lot different if either of these things occur.