Yeah I /purge on assist off of my tank and just always see Nothing to dispell. From what I read Purge is really only good on certain bossmob fights and PVP. My Priest uses /targetparty /cast Dispell Magic but pretty much everything you see on toons is non dispellable. With the cleanse totem poisons and diseases never stay on that long. Again it seems dispell is only really useful on PVP/Boss mob fights.

So for the most part they seem to be pretty situational and not useful for general grinding or clearing trash mobs.