well heres one that will stand up and say its great, if you cant see the use of a 6 second mini evocation that still allows you to move and reposistion yourself, with the added bonus of a shield wall effect incase you pull aggro in heroic/5mans then its your loss.
Dispersion is not a fix for pulling aggro... why did you pull aggro in the first place? Dispersion will be a very handy talent in raid encounters where there is significant or random raid damage. We already saw in TBC bosses able to do huge amounts of damage to random raid members and half the battle in those encounters was staying alive.
I personally believe allowing spriests to cast during dispersion is worth testing... at the very least, perhaps have dispersion lock us out of the shadow tree but allow us to use the holy tree. I'm not exactly sure what the answer is but dispersion isn't as bad as a lot of people are making out because only a very small percentage of players actually experience true end game content and thus don't understand how valuable damage reduction can be.