wtf is mindspike for?
Plenty of stickies/guides that will give you a very elaborate explanation about the usage of MS.
In a word, burst.
It was very much irrelevant until the Shadow 4 piece set came out for the current tier. Making the combination of shadowfiend, dark evangelism and mind spike and mind blast a very potent burst over 15 seconds.
Obviously this was done by the developers in recognition of the fact that we lacked a burst damage mechanic and such a mechanic was necessary for some of the heroic mode encounters in Dragon Soul.
Prior to this tier, it was used basically as a way to remove DoT spells quickly from a target, such as on Omnitron. It was also common to use mind spike on trash.
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I'll use it for exactly what was started: Burst (on low HP tagets) Essentially, targets that will be dead so quickly that your DoTs ar ineffective.
2 Quick examples:
Adds on Ragnaros
Sappers on Blackhorn fight
MS-MS-MS-MB-SWD = quick dead for any of these. Most times you only need the first 3 MS then you have the MB instant cast for wherever you want to use it.
things you need down fast but not requires a long fight. i use it on non elite mobs