It's the classic double edged sword of the pure healer class and why Blizzard created the shadow talent tree. In most groups a cleric is kinda dead-weight but it's needed in raids so when you're established in a guild your guildmates will carry you but rando groups would rather have a shaman or druid.
The playerbase (including me) has been lobbying DPG for years to change this paradigm. My favorite option is to increase the healing debuff by 3x and the dmg proc of the Vow line by like, 20x or replace the proc effect with a straight spell / melee damage increase.
It's currently:
[62164/4961] Vow of Perniciousness
Classes: CLR/118
Skill: Alteration
Mana: 1650
Target: Self
Resist: Beneficial, Blockable: Yes
Focusable: Yes
Casting: 3.5s, Recast: 1.5s, Rest: 1.5s
Duration: 72m+ (720 ticks), Dispelable: Yes
1: Decrease Healing by 25% (v125, Before Crit)
2: Add Melee Proc:
Pernicious Strike with 200% Rate Mod
Text: You take a vow of punishment.
Book: Utility Beneficial/Combat Innates
You take a forceful vow that reduces your healing efficacy by 25% for 72 minutes and adds the chance for your melee strikes to trigger Pernicious Strike, which deals 9083 damage to your target, increases their hatred for you by 4337, and heals you for 14490 hit points every six seconds for 24 seconds.
[62167/4962] Pernicious Strike
Target: Single
Range: 300'
Resist: Lowest
Reflectable: Yes
Focusable: Yes
Casting: 0s, Recast: 1.5s
Hate: 4337
Recourse:
Pernicious Strike Recourse
1: Decrease Current HP by 9083
Text: You are crushed by a punishing strike.
Book: Direct Damage/Physical
Deals 9083 damage to your enemies, increases their hatred of you by 4337, and heals you for 14490 hit points every six seconds for up to 24 seconds.
My other recommendation is to just delete the cleric class and change all the clerics to paladins. I recently swapped my cleric in my box team to a paladin and have no regrets.