>less mana used on heals
By increasing your damage you receive overall less damage from your target as the time to kill your target decreases in addition your Healer do not need to spend additional mana for heals.
Light at the end of the Tunnel!
If we add everything up we got until we found out that we only need as much HP (eHP) to survive until we can heal the received damage. We also found out that increasing our DPS as well as our Mitigation is increasing not only our survivability but also giving the Priest a chance to add even more DPS in the fight. Of course we want to have some extra HP in terms something happens and we have to wait for our next heal (Situation Number One) but we can also increase our eHP by increasing our Mitigation instead of our HP (making Situation Number Two unlikely to happen). However we cannot put Mitigation in our Stat Growth (sadly/only in PvE (PvP Defense)). We could put points into HP increasing our eHP or we would add points in All Attack decreasing the damage received (Mitigation).