Haste isn't low on value because of auto-attacks/km. It's low on value because it's capped quickly for rune regeneration seeing as we primarily use single rune abilities on live. Haste will be a lot better in Legion and potentially pre-patch due to it doing numerous things. More AA which gives us more attacks that can potentially crit, it reduces our GCD, and rune regen. Considering in Legion at least(the 4pc makes me think we will basically spam FSc the entire time aside from the rare spots we don't have KM up) We won't be sticking to single rune rotation, so naturally haste will be a lot more attractive. However I think there will be breakpoints instead of just a simple crit>haste>mastery.
- - - Updated - - -
I did it on a template character and breezed through it, then did it on my own copied character and struggled until the final part.