It's just another option. Whether it is useful for you is based on current gear level and what kind of spell rotation you use. The math and theorycrafting is difficult due to lack of direct comparisons, but in general go for haste if you're undergeared, BUT still go for spell crit if you have mana issues.
elitistjerks.com wrote:
The usefulness of haste is a contentious issue. While it increases DPS via more casts, some argue that you only see the gain from this if you get an opportunity to make that extra cast (ie: you would need to make 100/haste% casts to see extra cast). It is worth noting that while this is correct, it also reduces the changes a spell will be affected by interrupts/pushback.
Generally speaking, if you are undergeared, cannot use a LB/CL rotation, or do not have 4pc T6, haste would be useful. High gear levels and the ability to use CL in your cast rotation will count against using haste [Note: in 2.4, CL will NOT count against haste]. Also, it is important to notice that a high number of haste items are poorly itemised, lacking crit for example.
From
Elemental Shaman Bible. Keep in mind that spell haste WILL lower the GCD in 2.4, making 1.5 second cast spells benefit.
This graph will show what kind of haste:dmg ratio you will get in 2.4 using a 3LB/1CL rotation for varying amounts of haste.