You're not alone in this question - a lot of people are asking it.

Mind Spike has gone through some significant mechanic changes. Currently it is a 1.5s cast with no CD that clears all your DoTs on the target, consumes Shadow Orbs, increases MB crit% and is a bit stronger than MB (IIRC).

Blizzard kind of painted themselves into a corner in its development. If they make it too strong, SPriests will always use it, if they make it too weak, we'll never use it. If it takes too long to cast, it'll bugger our rotation and won't be useful as a 'nuke' - however if it casts too fast it runs the risk of replacing MF. So Blizz it trying to pound the square peg of MS into the round hole of a nuke that we can spam that won't replace our default filler spell (MF) or our default heavy spell (MB).

Beta's beta, who knows what they'll end up on. So far most SPriests seems to stay well away from MS since I think most are of the opinion that needs to be changed. I don't envy Blizz the design on the spell though, it's like trying to thread a needle with a shoelace.