Someone made a post on Reddit (I can't find it right now) mentioning that his DK off-tank was generating more threat than him, and having to throttle DPS as a result. He wanted to know what he could do to improve his threat generation.
I wasn't really satisfied with the answers that were given. In my opinion, the correct answer would probably have been "nothing". This is because improving threat generation is most likely a tradeoff, with additional survivability as the cost.
Almost all decisions that people spend any time considering involve tradeoffs, balancing of multiple competing values. This is necessarily true, because if a possible action has no downsides then it will be obvious that it should be done. An example of this would be enchanting your gear - some weapon enchants are better than others, but none of them will make your weapon worse.
Tanks gear and spec for two things - threat generation (of which DPS is a subset), and survival. Of course you can improve both by getting better items, but I would assume the tank already had the best gear that was readily available.
This means that, assuming he didn't have any items or talents that were obviously bad, increasing one of these values would have meant a decrease in the other. And it is almost certainly not worth it to decrease a main tank's survivability by any amount in exchange for a slight boost to the DPS of a single off-tank.
I wonder if people would have reacted the same way if the question was phrased as "what part of my build is worth losing so that my OT doesn't have to throttle DPS"? The question is really equivalent, but it makes the fact that a tradeoff is involved much more clear.