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Spell haste isn't quite as bad as you might think.
In an ideal 30 second cycle a shadow priest should cast 2*VT, 1.25*SWP, 2.5*SWD, 5.4*MB (assuming 5.5 imp MB). That lot adds up to 16.7 seconds leaving 13.3 seconds for other stuff.
Now in the real world there is lag so you won't be casting all those spells quite that quickly, however in the real world the cooldowns don't overlap very neatly so you won't actually fit that many of those spells into a 30 second window.
In practive I typically find that I'm spending about 40% of my time casting mind flay, so 40% of my cast time is gaining a benefit from spell haste.
Running the numbers through the various simulators available and at 1200 damage you get something like 3 spell haste = 1 damage, which is big enough that it can't be ignored.
Since you mentioned crit, the same simulations show that 6 crit is worth about 1 damage (assuming using SWD - a bit lower without). Again not awesome but big enough that it can't be ignored completely.
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