Why can't we vanish during shadow dance?

Is there a reason for this. It is really, really annoying to not be able to vanish while shadow dance is active. It is fine in PvE I suppose but there are times when you need to vanish defensively even though shadow dance may still be up. There is no way to cancel shadow dance so you can't even do it that way.

You can talent to have shadow dance active for up to eight seconds and if your healer gets CCed you are stuck riding it out. It is just really poor design that you can't use your defensive cool down when needed.