The problem with disc using haste, is it doesn't increase our raid utility which is what disc is mostly about.
The ability to absorb, and increase the effect of our absorbs.
You can say haste will put absorbs up faster, but in a much weaker state. While haste might be a good stat because its a flat throughput increase, if you wanted a flat throughput increase I might recommend going holy at that point. You never want to flat out nerf the amount any of your absorbs can do, you want to always enhance it.
And unfortunately haste just doesn't do that as much as mastery/crit right now