That quickened Heal won't save them in a lot of situations. Nor does a quickened Heal have the effect of totally resetting the target's aggro. If a 450hp melee pulls Horoth's aggro, you will be sending them a lot of quickened Heals, Heal scrolls, and other less efficient spells to try to keep them alive until aggro can be switched back to someone that is better specced for tanking.
Most Clerics use the capstone before it is needed or not at all. The capstone should instead be used like Unyielding Sovereignty - held in reserve until it is really needed, then fired off at a critical moment in a fight.
At the least it prevents a death and SP-intensive rebuffing (100 SP saved in fights like eVON6 that aren't buff intensive, much more in fights like LOB where Deathward, FoM, Haste, GH and Elec Resist are all needed). Often it will save more than that.