Accuracy does not calculate into healing at all, it's a stat that calculates against enemy status in order to calculate your and their hit rates.
Enemies status are not calculated with Healing as it is a player to player move, so as such is impossible to miss with Healing anyway.
Tanks need it slightly more than DPS because they hold aggro, which means they are attacked more so they need enemies to miss more, which requires higher Accuracy ratings.
Accuracy has nothing to do with defense. At all.
It only has to do with you hitting the mob. That's it. Period.