Even if a pushback prevents the final tick from landing, this is still a more mana efficient heal than [Greater Heal], and will still heal more than [Flash Heal]. Pushback can only prevent one tick, as it can only cancel 50% of a channeled spell.
The instant first tick of this spell can save party members who would otherwise die while waiting for [Flash Heal] to cast.
Patch 5.2.0 (5-Mar-2013): Evangelism's stacking effect now also increases the healing of Penance.; Penance damage and healing has been increased by 20%, now a cooldown of 9 seconds (was 10 seconds), and has a range of 40 yards for both friendly and hostile targets (was 40 yards for friendly and 30 yards for hostile targets).
Patch 4.0.6 (8-Feb-2011): Penance mana cost has been increased by 7%, but healing has been increased by 20%.
Patch 3.2.0 (04-Aug-2009): Cooldown increased to 12 seconds, up from 10 seconds.
Patch 3.1.0 (14-Apr-2009): Penance can now be cast on the priest himself and the damage has been increased.