Why do I see so many caster dps builds using spirit instead of intellect?

[Narmok]

Intellect has always been the stat that casters want to increase dps, so why am I seeing so many builds like pure shadow and astral bomber where people tell me to use spirit gear? Are there any builds where you should use intellect gear?

[Battlecrafter]

@Narmok I believe they take Intellect as their main stat and then gem and gear with as much spirit as possible and use abilities and talents and also gear that enhances/multiplies spirit with those that increase spell power/per spirit at a higher percentage than intellect.