Right now I have to question why Blade Storm and OS have a dot component as opposed to higher up-front damage. The only purpose they seem to server in PvP is that they can offer clense protection for shatter (as clenses remove the oldest dots first). However because of the high damage the Shatter dot does and its long tick time you generally want to get it up on the target asap so for me that theory sort of doesn't work :S
I don't see these DoTs as a cleanse protection for Plasma Brand, but rather as a "you shall not stealth before me". Overhead Slash and Blade Storm being different attack types (Melee & Force), it allows me to use them as a counter of respectively Force Shroud and Evasion, because I will be able to DoT them again before they try to disapear, and most of the time it screws their vanishing move.