OK a bit of explaination needed here
For a mage DPS is dealt only when your spells hit.
with 50 hit rating, your looking at a miss rate on boss mobs (which are the most important for a raid) of around 13%.
Lets assume that miss rate is a statistic and not a chance means for 100 firballs 13 of them will be complete resists. This makes KT's theory very plausible.
The real mechanics however is like this, every fireball has a 13% chance to miss given that you have a 80+% chance to hit it. Inherently, you can never ensure that you will hit all your spells as the game forces 1% miss.
This is based on the 3 outcomes rule a number of people believe is the way WoW does it combat calculations. The same applies for melee dmg. Each swing can either crit, hit or miss. Only one out of three, no such thing as crit but miss....
glancing blows, parrys and blocks belong to another set of 3 outcomes that has to do with damage migitation. Same applies for spells, each spell (binary ones) can only be partially resisted, totally resisted or missed by the caster. All are reflected under the combat log as "resist".
As a mage you need to balance your stat around
getting your spells to hit
making your spells hurt
making your spells hurt really really bad (crit).
for a raiding mage, spell misses hurt really bad since your nukes are casted. DPS is serverly lowered by misses, since you wasted all that time casting a missed spell and also you wasted mana. OOM mages deal ZERO dps. Less missed spells means more bang for your buck and you will perform better.
This is the reason why I would cut my damage down to crit more to the range of dropping 10-15 spelldamge for say 14 more crit rating. Any more spelldamage dropped is detrimental since I crit more often but for less. Spellhit also plays a important part. If there is more spell hit to be had then more is better. Sure I drop crit for that but I can always make it up with less misses.
item budget wise, crit rating, hit rating both cost the same so its really down to you and your play style. My raid leader would not be pleased with me if I can crit l70 mobs like a **** but can't get enough hits in on Gruul.