Well, you get 9% crit from must-have type talents in the fire tree, thus the discrepancy between frost and fire critical ratings. IMO if you are talking instances like Kara, SSC, TK, Mag, Gruul, etc, you should be stacking +hit before +crit. Also, as fire, crits give back mana, so mana efficiency is another reason for the need of high +crit.
HTH