Yep I said it. I see so many people recommending for disc priests to gem spellpower/haste and favor haste gear over crit, etc etc. In 25 man the majority of your healing should come from pw shield, pom, and penance. Typically I may cast flash/binding heal like 6-10 times in an encounter in 25 man since the aforementioned spells are more powerful for 25 man, and each of those casts is typically hasted by 31% with talents. How does haste benefit those spells? Well, for pw shield it doesnt benefit you at all - you're at 1 sec gcd with hardly any haste from gear. PoM? Same story. Penance? Your penance will cast very slightly faster, but with 31% haste from enlightenment + borrowed time pumping haste is pretty much useless on penance. Your flash heal will cast slightly faster but you'll still be GCD limited - your throughput will not increase. With 625 haste (much more than I need btw) my flash heal is already well under 1 second with borrowed time.
So when is haste useful? It's useful when your spamming flash/binding heal - meaning you don't have borrowed time up, which typically only happens in 10 man and VERY occasionally in 25 man (northrend beasts, particularly when it was new content).
I see again and again people recommending to new disc priests to drop their spellpower/int gems for spellpower/haste, which if they play the spec correctly will offer next to no benefits in 25 man raids. People argue that "disc priests never go oom." Yes, yes they do. If you are not going oom then you are not casting on every GCD - watch my video I posted of Lichking 25 a few days ago, I was getting mana back in every possible way and mana was still an issue. Faction champs heroic, another fight where I have had the #1 disc parse for a period of time, is another fight I frequently go oom on while receiving innervates. Intellect is not a waste at this point even in all 264 gear - maybe, just maybe at 277 ilvl that will change, but right now it really hasn't. Adding haste to your gear for 25 man on the other hand is adding no benefit to the majority of the spells you should be casting. Ideally you should have a bunch of haste gear for 10 man that you can switch to, but for anyone who cares mostly about 25 man, I'd say take crit over haste, and for most fights, even take spirit and mp5 over haste. That is all.