At some point Blizzard let us in on a design goal of theirs, which is to have talents be worth about 1% each. 1% more damage, more healing, more whatever. Of course there would be exceptions. It would be pretty boring if every talent were the same. But that is the baseline: more than 1% means a good talent, less than 1% is a weaker talent. Often you need to take weaker talents on the way up to the good ones.
Now discipline priests, I feel, are particularly bad when it comes to having good and bad talents, or I suppose just when it comes to having bad talents. Discipline Talents do not improve overall throughput as much as perhaps they should. Or it might be that discipline talents are about where they want things to be and most other trees are overpowered (the average talent a discipline priest takes increases effect by just over 1%, the good ones being balanced by very bad ones).
Generally, I'd rather not be taking a talent that has less than half a percent effect on my output. Those seem like very weak talents that I should be looking for something else to put in place of. Unfortunately, that is not really an option. A lot of talents such as Improved Inner Fire are just stepping stones with no other options available (18 spell power per point!). But when it comes to the top of the discipline tree, there are lots of good options, and I shouldn't be left scooping up the filler.
And that's where Grace comes in. According to the stats provided by www.quickarmory.com 94% of discipline priests take this talent and 82% take both points in it. This puts it on about even representation with Divine Aegis.
My spreadsheet works with my actual casts. I wrote a crude log parser (I call it crude because it's an excel macro) to get data from my combat logs and that data feeds into the spreadsheet to give me my own actual healing per second, mana use, and how that would be affected by changing stats and talents, so I know how good these talents are relative to one another. Divine Aegis increases healing done (counting absorbs as healing) by about 2.7% per point, so it is a very strong talent indeed. That second point of Grace is worth a whopping 0.3%.
That's right, it is total garbage.
In order to understand how bad Grace is, you have to know a bit about how Grace works. Take a moment to read the description of it's effect and I'll go on. So the second point of grace gives you a 100% chance to stack the effect, while the first point gives you only a 50% chance to stack the effect.
Imagine I focus most of my heals on a single target. That target will have Grace stacked up on them a lot of the time. Regardless of whether I have one or two points, I'm pretty likely to maintain a three stack. After all, if I am focusing heals on someone, 15 seconds is a very long time to hit with enough heals that the chance it doesn't renew becomes awfully small. But what if I throw a Penance or Flash Heal at another target who needs it? If a lose my stack, it will take longer to rebuild with only 1 point than it will with 2. On the other hand, with only one point, there is only a 50% chance I lose it from my main target in the first place. If it doesn't get applied to the target of my heal, it doesn't drop from the target of my previous heal.
In addition to all the real things going on, my parser puts a "Fake Grace 1" and "Fake Grace 2" buff on people I heal. The numbers represent the number of points. So whenever I cast one of specified spells I put Fake Grace 2 on the target, increment the stack if they already have it, and remove the stack from anyone else. I also flip a coin to see whether I do the same with Fake Grace 1. Doing this allows me to get results for the average number of Grace stacks that I would have on my targets for each spell.
As you can see, the difference is not that great. For some spells it works out to be a lot more than others. It isn't surprising that Greater Heal ended up with the same stacks for 1 and 2 points because I only have 2 GH casts in the logs I am using for this and so a couple of coin flips would make it come out the same. As you can see, the difference in average number of stacks is not double when you put that second point in, in many cases it is increased by about half or less. Every stack is a 3% throughput on that spell. So for Penance, which goes from 0.77 to 1.19 stacks, that second point is about a 1.26% increase in the effect. For Flash Heal, whose average goes from 0.32 to 0.47 stacks, that's a 0.45% increase in effect.
Of course the first point in Grace is not that overwhelming either, the reality of that point's effect on my healing is about a 0.56% increase. Almost double the effect of the second point, but still a pretty bad talent. The thing is, the first point can be a 9% increase in healing when you need it most. If one person is taking really massive damage and you are chaining heals on them then Grace does what it is supposed to, and does it about as well whether you made the half or full investment in it. There is a situation in which the first point of Grace really performs. The second point of Grace, however, performs quite randomly. It is not particularly better in the situation that it is designed to work for, and when throwing heals around the raid they are almost the same.
What is so sad about all of this is that I have two points in Grace. When it comes to spending those last few talents points I'm just scraping the bottom of the barrel so badly that a 0.3% increase in my overall healing, doled out at random, is better than my other options1. Discipline priest throughput is really, really weak compared to other healers and I wish that this talent was doing what it looks like it was meant to do, but for now there is nothing I can do but take it anyway.
1. Spell Warding is the next best thing I could take from a raid healing perspective. Reducing the amount of magic damage I take by 2% is about 86% as good as that second point of Grace is for improving the total health of the raid.