Do you honestly think you are going to be able to get 5 stacks of Vengeance up on multiple mobs like the ones in Thorim's arena, or the ones that spawn with Razorscale before they die? Lets see, spending several seconds trying to get 5 stacks on multiple mobs and THEN doing 48% weapon dmg OR doing 36% right off the bat. It's a no brainer. Heck, you probably won't even get a full 5 stacks on the main mob you are targeting every time since they die so fast, much less all the rest of them getting AoE'd that you aren't targeting.