Honestly, Giantbane, you're under rating Inner Beast, even for as highly as you're already considering it. That 6 seconds of reduction is up more like every 15 seconds, not including the use of skills like Vengeance, Berserk, and Infuriate; and being able to activate it more frequently is worth a hell of a lot more than the longer duration of Dark Mind, since it means that you can use Inner Beast to block way more individual tankbusters.

You could probably have Inner Beast fill in for several of those DRK cooldowns, even.