Plenary Indulgence was always useful if you aren't one of those healers that feel the need to 100% everyone immediately after every raid-wide.

WHM is a job that is designed around two things: regens and and burst heals over short periods.

Plenary Indulgence (as well as Cure III and Thin Air) are designed around this play style. Where other healers work by either preventing damage or healing it up immediately via pre-set stars, WHM is about ignoring damage until right before it needs to be healed. This compiles your heals from the previous raid-wide and the incoming raid-wide together into one heal session, all of which can be covered by Thin Air, and all of which contributes to Confession stacks.

Example:

After the first Tidal Wave on O9S, do not heal. Instead, wait a couple seconds, use Thin Air, and stack on the stack point. As Chaos begins to cast Umbral Slash, cast Cure III twice to 100% everyone leading into Umbral. You now have two PI stacks. You can then cast a third Cure III or Medica II as Umbral's damage lands, netting you three PI stacks within Confession's 10s duration, allowing you to follow with a 3-stack PI to heal the rest of the damage.

Because of this, the 30s Confession was never needed. Now that they have caved and extended it, however, more windows open up where you can use Plenary Indulgence at 2 or 3 stacks without compiling heals, or where compiling heals isn't an option.