Holy Pally: why are you gemming all intellect?

going to try and give a straight forward and solid answer as to why i and many other holy 'light' pallys gem nothing but 20 intellect instead of sp, sp/int (ametrine), or int/mp5 (eye of zul) gems.

first thing to mention is that in this case, socket bonuses are not that important. i know some people like to match them all but ultimately, gaining 5-7sp is not worth losing 10int (keep in mind that 20 int gives you about 4.8sp from talents, see below)

- Mana regen
Divine Plea: 25% mana regen over 15 sec every minute.
Replenishment buff: gain 1% of total mana per 5 sec during combat
Judgement procs: 4% mana return with seal of wisdom active (~57% chance per hit to proc). often ~9% regen a minute if done right.
Mana tide totems from a shaman will also scale with your total mana (but you cant rely on this to happen)
all of these are based off percentages of your total mana. therefore the more mana you have, the more you will gain from each of these.

- SP splash effect
due to talents, 20% of your intellect adds to your spell power. this means 20 int = addition 4sp.
with 10% int gain from talents, that's actually 4.4sp, and with kings (another 10% int) its 4.8sp per 20int gem (which is just about as much as my 5sp socket bonus).
if i have 2089 int unbuffed, that's 417sp gained. with kings its 2289 int which is 460sp gained. 21 sp gems would give you 483sp. i know a pally with all sp gems would have more total sp then a int gem pally, but he would not be able to cast anywhere near as many holy lights and would be more suited for flash of light spam (holy light spam is superior in ICC).

- Healing hard and effective
some fights you need to lay out holy lights left and right (ex. marrowgar 25, festergut 10/25, especially heroic raid modes) to effectively heal it. more mana results in being able to cast more holy lights overall, out healing pallys with sp gems (or just less mana in general) that cant do as many big heals in long fights.
an example i really like is festergut because during that fight even on 10 man mode i pretty much have to do non stop holy lighting when 2 healing it due to high raid damage followed by high tank damage (i do get some flash of lights off here and there) and even with 41k mana buffed, a holy pally has to do good timings on divine plea every time its off cool down or else they will go low on mana. fights like that and many others is where all int gems excel for holy pallys especially when a divine plea/illumination mistake can make the difference between going oom and having just enough mana to heal until divine plea comes off cooldown for 10k+ mana restore.
in my experience progressively healing icc at first with ~32-33k mana, now 44-46k (depending on trinket selection). every bit of mana gained did make a nice difference. when i got an extra 1-2k mana, i could certainly tell not only in my ability to heal for longer, but more effectively heal because i could cast more holy light as my mana pool went up.

because of the int gemming you will often find your self with less sp then other healers gemming sp and stacking 2 sp trinkets, but due to the high crit chance and holy lights still overhealing most of the time, you should be easily healing just as good as them if not more as tanks (your main heal target) do take the highest amount of damage over anyone else in the raid. for the longest time i've healed with a int and sp trinket, recently trying double int trinkets and found that even on festergut 25man (my highest hps fight not counting dreamwalker), despite having a little less sp, my hps was not hurt negatively, however my mana was looking a little better during and at the end of the fight.

update july 2010:
one thing important to add is you don't always have all the mana regen buffs in raids. judgment procs and divine plea will always be there, but mana tide totem is not guaranteed, and replenishment (which is huge) may not always be in the raid. with out mana tide and replenishment you will indeed see mana going down a little quicker when casting holy light. having more mana to burn is unquestionably valuable at this time if you intend to keep your hps high from holy lighting instead of having to slow down with flash of lights.

little more info on meta and prismatic gems at http://www.theholypaladin.com/healing-in-3.3-gems.php