First of all, Holy Shock is instant cast -- that is its most important virtue. Outside of the long CD of Lay on Hands, it's our only direct heal that we can cast while moving. I have gotten into the rhythm of using my Judgement spell any time I have to move, as well as using those few moments to refresh Beacon or Sacred Shield. I realized that by not using Holy Shock every time that I moved, I was missing the opportunity to increase my healing done. It's not that I was never using Holy Shock, because if I saw someone dip into the red zone, I already had HS bound to a reaction key to save their life. The issue was that I had a mental roadblock that told me not to use HS, because the mana cost was too high and the healing was too low.
Holy Shock's vitals
Due to the amount of healing done and the speed of doing that healing, Holy Shock finds itself next to Flash of Light when grouping holy paladin abilities. In a raid environment, with Judgements of the Pure active, both spells are simply one GCD abilities. Holy Shock scales with spellpower at a 0.81 ratio, while Flash of Light scales at a 1.0 ratio. This means that for every point of spellpower that I add to my gear, Holy Shock increases by a smaller amount than Flash of Light. However, at our current spellpower levels of around 3,000 SP, Holy Shock will still heal for a greater amount than Flash of Light. Even though its scaling is worse, the base healing value makes up for that deficiency. FoL actually won't overtake HS in term of healing until 8,500 spellpower, which may not even be possible to achieve in the finest gear of Cataclysm. In short, Holy Shock heals for more than Flash of Light.
When we start to look at mana costs, we all know that Flash of Light is so crazy efficient that you can chain cast it for 10 minutes without running out of mana. Holy Shock, on the other hand, is right in the middle of FoL and HL in terms of mana cost. Once I started thinking about the fact that HS actually costs a lot less than a HL, I started realizing that I could add more Holy Shocks into my rotation without worrying about mana issues. I take all the proper precautions in a fight, like meleeing with Seal of Wisdom and using Divine Plea whenever possible to ensure my mana is the highest that it can be. I rarely have mana problems that aren't solvable, and so adding in a few spells that are actually cheaper to cast than a Holy Light shouldn't cause my any issues with my longevity. Holy Shock's mana cost is low enough that we can safely use it without worry.
While the casting speeds may be out of order, Holy Shock can actually fill the niche between Flash of Light and Holy Light fairly well. Obviously its cooldown prevents it from being a bread-and-butter ability, but when using the eponymous glyph, we can work it into our repertoire pretty easily. I've been trying to use Holy Shock for raid healing any time that I would've used a Holy Light instead. A great example is on the heroic version of Blood Queen Lana'thel. There is a constant AoE damage pulse that is unlikely to kill a player directly but we must keep everyone topped off so that her other abilities don't finish them off. If a player is hovering at 50%, I could've previously spent the time casting a Flash of Light which wouldn't have brought them up to full HP. I also could spend even more time and mana using a Holy Light which will do a lot of overhealing, and risk having the player die to some other attack in the time it takes me to cast HL.
Holy Shock has a niche
Instead, I can simply fill that gap with a Holy Shock. Not only do I reduce the chance of that player dying to a Bloodbolt Whirl while I'm casting, it also cost me less mana than the Holy Light option. Once we factor in the perks gained from Infusion of Light on a critical Holy Shock, it's clear that HS fills a specific role: bigger than FoL, and faster than HL, with a mana cost that is right in the middle. While I still won't use Holy Shock to heal a tank unless they are in the most dire of situations (and even then I'm using my Divine Favor macro), I do see the value in using it to help me handle raid healing a bit quicker. It can also help prevent "heal sniping," since the player will instantly be topped off in the other healers' Grid windows.
Holy paladins already favor using the biggest heal possible regardless of mana due to our powerful regeneration and huge base mana pool, so using Holy Shock in place of a Flash of Light every 6 seconds fits into our existing healing mindset. In Cataclysm, when we move to a 3-heal model, Holy Shock will likely find itself in the same position, right between our fast and medium speed heals. Its mana cost has to stay low in order for us to want to use it, so its cooldown will be used to prevent us from abusing it too badly. Since we are receiving this spell as a baseline ability in Cataclysm, I am hoping to see something interesting done with the talent point slot that it used to inhabit. Perhaps some effect where a Holy Shock onto a Beaconed target causes the heal to replicate to several nearby targets? Only time will tell what plans Blizzard has for Holy Shock, but I hope that they keep it in mind when designing the holy paladin healing model. We need to keep an instant heal in our toolbox, and Holy Shock is the best candidate for the job.
The Light and How to Swing It (Holy Edition) is dedicated to helping holy paladins become the powerful healers that we're destined to be. If you're new to the paladin's healing ways, you can learn the ropes with our Holy 101 article. We also have information on how to keep a tank alive, how to heal a raid when necessary and how to beat the GCD. Tanking is a job, DPS is a craft -- but healing is truly an art.