I know. You don't come here for math. My maths, they are bad. BUT! The other way to look at this, is if even my mathtarded self can figure this out, you can too! Hurrah!
I figured the best way to settle the socket bonus debate was to simply math it out. So, I turned to BobTurkey for the source numbers (because that is definitely waaay beyond my capabilities) since I trust him, and figure his weight values are pretty good.
So I could try and explain what I did, but spreadsheets are way cooler. Please note, these numbers are for a Discipline Priest - the numbers for a Holy Priest are different, and you should go clicky the link to BobTurkey's Theorycrafting House of Pain to figure it out (also, he does not call it House of Pain - that is merely a reference to my brain hurting).
This is what my current gem setup looks like. As you can see, Spellpower was my priority. Whenever a socket bonus required a blue socket, I used a nasty, dirty, death knight birthing Purified Dreadstone. Whenever a socket bonus required a yellow socket, I used either a Reckless Ametrine or a Potent Ametrine, depending on what boss I had just wiped on and current prices on the AH.
As a side note, if you're wondering why I choose Spirit over Mp5, the answer is simple. Both are blue gems - however, Spirit scales with Blessing of Kings, whereas Mp5 does not. In addition, though Mp5 and Spirit are roughly equal in value to a Disc Priest, Spirit is far better for a Holy Priest, and I would like to be able to use the same gear when I run Holy.
All in all, it's not bad. The total item value number thingie is approximately 454. I have no idea how BobTurkey got his numbers, because everything he wrote about that translated into "BWUH??!" in my head. Since I primarily heal the MT in ICC, I might argue that Haste has a slightly higher value to me, but I can in no way quantify that, so I don't even want to open that can of worms.
Okay, now you've seen teh maths about my current gem setup. What if I ignored all socket bonuses, and simply put a Runed Cardinal Ruby in every socket? Please note that you still need to make sure to fulfill your meta gem requirement. Since I am using Ember Skyflare Diamond, I don't even worry about it, but if you use something different (and many do), that is something you should consider.
Here is the magic spreadsheet I generated. If you want to try this method, please note that you need to check your socket bonus requirements. Unfortunately, NONE of my current gear had a socket bonus that could be satisfied purely by Red Cardinal Rubies. That is why in the (Bonus) row, every calculation is zero.
See that super yellow and red number (that hopefully matches my website)? That number cries "HAHA UR RONG NOOB" and taunts me with its additional 6 points.
The differences between these two gem setups come down to this: Ignoring socket bonuses gains me 34 spellpower, at the expense of 30 crit, 20 haste, and 20 spirit.
Unfortunately, BobTurkey has not yet provided weight numbers that quantify the amount of hair I will rip out and gold I will spend and expletives I will shriek trying to change my gems YET AGAIN. If I were a min-maxer, I would probably care more. If you are a min-maxer, maybe you DO care, so there you have it.
Does that mean that all Spellpower gems is the best solution? Not necessarily. I'm sure someone much smarter than me can come up with a formula that will tell you the best gems to pick, based on your current soft caps and raid buffs and all that jazz (I believe it's called RAWR), but now you're just being picky. I threw together an alternative gem selection that would be better than both of the previous, just to give you an example.
So here you go. If I assume that BobTurkey's numbers are ideal, the gemming for crit, spellpower, and spirit is still superior to gemming solely for spellpower. The magic number 468.594 tells me so.
It's actually a pretty simple spreadsheet to throw together if you feel so inclined, but this is the part where I give you the disclaimer.
I have the utmost faith in BobTurkey's theorycrafting abilities - but nothing changes the fact that this is theorycrafting. It assumes certain variables. If you're raiding without Replenishment for whatever reason; If you are primarily raid healing and not MT healing, and find yourself nearly always under the influence of Borrowed Time; If you don't raid and stick to 5 mans; If you raid with 24 Druids - all of these factors can change the value of these numbers for you.
If I think about all that too much, my head esplodes so I just... don't. Contrary to popular belief, I like numbers even if I am not as capable as BobTurkey and Codi in figuring out what the best numbers are. But I am also the kind of person who critiques others' research and statistics as a hobby, so I am all about encouraging people to read the numbers with a healthy dose of skepticism and realistic consideration. Dont drink and theorycraft!
ETA: One time, I made a post on the forums that some other priest disliked, so in order to pwn me hardcore, I guess he checked me out on armory in an effort to find something glaringly wrong and nooby. In the end he pointed fingers and lawled at my gem selection. At first I was insulted, and then after thinking about it, I was rather flattered. Aww, that's so cute. You couldn't find anything to criticize me for, so instead, you turned to the default criticism that you can use against anyone: YOU GEM LIKE A NOOB.
The fact is, there are several different "gemming philosophies" (though I suspect I'm one of the few people who would call it that). What people have a tendency to forget is that these gems are very small amounts. They are nice additions that you don't want to skip, but at the same time, choosing a Reckless Ametrine over a Potent Ametrine doesn't actually make that big of a difference.
The way you choose your gems will differ depending no gear level, what you are currently healing through, and whether or not you are pushing up against the hardest content available. If you are trying to conquer hard modes, you will likely have to do some min-maxing. Otherwise? It doesn't really matter, as long as you pick stats relevant to your class and spec for gemming (i.e. no strength for priests, no hit for healers).
Why am I adding this? Because I don't want you to think I am suggesting everyone should put together a spreadsheet and try to figure out the "ideal" gem selection based on stat values. My point in this little experiment was merely to show that it is completely legitimate to gem for socket bonuses - and you can even use math to back it up if you like.