Just in case there was any lingering doubt, I can confirm that 0/10/51 is not a good DPS spec. I had read some praise of it on Brig's forums, and so decided, in a disastrous move, to test it for levelling around halfway to 71.
The real problem with it is simply the setup, and in that respect, trapping has of course always been slow. If I'm farming in a scenario where I don't care how much time I spend, that's fine; and especially considering that in Outland after I got to 70, pulling threat was also a real issue. Although it's true that once the trap goes off, mobs will die with that spec faster than they do with 0/31/30, as mentioned, it's the setup that's the killer.
I recommended trapping (and I still do) against heavy armor, when you don't have a lot of armor pen. However at the moment, a) I'm sitting on around 3% armor pen, b) the early mobs in Northrend don't have much armor, and c) because armor isn't an issue, I can kill mobs before they cross the 40 yards to reach me even if I pull threat, and so being able to start shooting as soon as I send my pet is more important to me.
It is also true that mana efficiency is much better with 0/10/51 than with my DPS spec as well. However, even though with my DPS spec I'm having to drink every four mobs, I find that trapping is that much slower that even with having to do that, I still save time. I will probably be going to bed in less than two hours from when I write this, but with a bit of luck I might get to 71 before I do.