When and why to use mind spike

I am unsure of when to use mind spike.

I've noticed that my shadow priest rotation often boils down to the following:

shadow word: pain

vampiric touch

mind blast

devouring plague

vampiric embrace

mindbender

shadow word: death

I'm not sure where mind spike should fit, I do know that it cancels all damage over time abilities so I can't think of when to use it.

Also, is there anything I can do to improve my rotation.

Is there any difference in the approach I should take for solo, raid and dungeon play?