I've noticed that with the overall increase in shadow priests gear that a lot of priests are stacking a decent amount of haste. In my opinion, the shadow priest priority (I don't like the word rotation as I don't think it fits) revolves around casting Mind Blast as often as you can (once every 7 seconds assuming 0 haste and 5/5 Improved Mind Blast). With 0 haste, you can cast 4.5 seconds worth of spells in the 5.5 second MB cd:
Mind Flay + DP, VT or SWD
or
DP, VT, and SWD
You can also cast 6 seconds worth of spells during the 5.5 second cd:
Mind Flay x 2
or
Mind Flay + 2/3 of (DP, VT, SWD)
You can also clip mind flay when MB becomes available. For the purposes of simplifying my argument, I'm going to ignore this possibility as I don't think clipping is a very promising strategy anymore. This leaves you either wasting 1 second waiting for MB to become available or not casting MB for .5 seconds after it becomes available. The first option in my opinion seems stupid. Wasting an entire second worth of dps every 7 seconds is ridiculous. The second option is not ideal, but at least time is not being wasted. However, it is being used inefficiently.
With 300 haste you get 9.12% faster casting speed, which allows you to cast 6 seconds worth of spells in 5.5 seconds. This allows you to virtually always either be casting MB or have it on cd. Now I'm not trying to say haste is greater than spellpower, crit, or hit; but as it often comes as part of an item's itemization, it is still useful and not wasted itemization.
Now, my problem/question is why would you want more than 300 haste? In my opinion, having over 300 is haste is partially wasting itemization. If you have more than 300 haste, you're having to sit there wait for mb to become available or leave it available while you cast another spell. I do recognize that having more haste decreases the cast time of MB and thus increases your "MB rotations", but you're still hindered by the 5.5 second cd and thus the haste is only partially used.
I understand a lot of gear has haste on it, and that it's not always easy to find a replacement that has less haste on it, but having 400-450 haste is essentially wasting 100 haste worth of itemization which could be crit or spell power. I've never seemed to have a problem staying at 300 haste. Am I missing something? Is there a valid reason to have above 300 haste?
I'd love to see thoughtful and even argumentative responses/thoughts.