Originally Posted by Xylia

Both of these skills require physical damage, which can be a hindrance if you don't have a significant amount of it in your team setup. In general I prefer to bring [skill]reckless haste[/skill] in place of one of these skills, especially in hard mode. Also, in hard mode mark of pain will cause your target to run away from the rest of the group, or cause the group to scatter due to AoE damage. While this is good in some places, it is generally a pain if you don't have a swarm of minions handy. The minions will body block to reduce scatter, and they will often trigger mark of pain so fast that everything will die before it can figure out how to run away.

Obviously if you were running a team of b/p rangers or something else heavy on physical damage, you might find some additional use for MoP, but I wouldn't recommend bringing it if you don't have 3-4 or more constant sources of physical damage. [skill]Splinter weapon[/skill] will also trigger mark of pain, which is an interesting combo.