How much damage does a moonkin do compared to another support-caster: the winter's chill mage?
Spelldamage calculators
Ungeared
Warlock 342
Winter's Chill mage 297
Boomkin 227
Midgeared
Warlock 661
Winter's Chill mage 538
Boomkin 455
Endgeared
Warlock 844
Winter's Chill mage 681
Boomkin 587
3% more chance to crit on an endgeared warlock would add 23 dps, times 4 its 92 dps. If we credit that to the moonkin, his dps goes from 587 to 679. This is still 80% of the dps that the warlock did before the 3% extra crit.
In short, no the extra crit does not outweight bringing another warlock. BUT wait! theres more
If you're playing a hybrid class and you're always performing just one role, you're doing it wrong.
The strength of a moonkin is that you are a hybrid. You can go dps on trash and some bosses, while swapping to healing gear on other bosses. You want to clear the instance quickly, you don't want healers with full mana on trash - let them do some damage. You have healers with full mana on some boss fights? Let one of them go dps. A moonkin is perfect for this.
You can support heal easily on fights like bug trio where healers have to be scattered all over the place. You can remove poisons on viscidus. How about healing on patchwerk and swap to dps on grobbulus, gluth and thaddius without ever getting another player into the instance.
A moonkin can be a good addition to a raid if you're smart about it.