Altho i am new to this game, i used to be a big theory crafter in WoW, crit chance is always good before u reach a certain threshold. But as some people say, if you dont crit , u wont see crit dmg. What good is it if u have 300% crit dmg, but your crit chance is only 10%. (probably most of u will argue u can get items easily to boost crit)
But, u cant really count how long each game lasts.... sometimes it lasts for 15 mins? sometimes an hour, and according to OP's math, crit chance will def give u an edge early and/ or mid game.
Thus.... ask yourself this, if u are fighting another champion alone, and they have already crit u 3 - 5 times in a row, leaving u to only 20-30% hp left, i think u would start considering running away.....
these crit dmg, (numbers in big red color) have a psychological effect of scaring people... :P
All in all, crit chance will always help u thru-out the game until u reach 100%, but in order to make crit dmg counts, you need to score a crit first. For me, i definitely wouldnt want to wait till late game and farm all the items to do a big crit. just my humble opinion..