I thought the Purified buff you got from orb world stayed on until you soaked a blue orb.
But the buff you get, spellID 146022, lasts for 5 seconds only and the describtion says "The process of Purification grants the caster immunity to all damage for a short time.".
This is from looking at logs at warcraftlogs.
What am I missing - is the true benefit from orb world getting adds to do unreduced DPS on?
Once you're purified, you're immune to damage for a few seconds as you shift back into the other world. That's to prevent you from coming up inside the beam and instantly getting cut in half.
http://www.wowhead.com/spell=144452
is the actual buff that adds healing for you. You get both when you come out of the other phase, the dmg immunity disappears shortly after.