Crit Damage runes are not recommended because they are very weak during the early and midgame. This is when you do NOT want to be weaker than normal.
Lets put it this way:
A tier 3 red crit damage rune adds 2.2% crit damage. If you hit for 100 damage and have 100% crit, this is equal to adding 2.2 AD.
A similar rune for AD adds .93 AD, and the scaling one adds 2.46 at level 18.
If you hit for 50 damage and have 20% crit, it is equal to about 0.22 AD which sucks.
Eventually when you hit for 200 damage at 100% crit, it is like adding 4.4AD. But really? Is 4.4 AD all that freaking fantastic for super end game? By then you crit for a thousand damage! Your runes arent that huge of a factor.
I personally take Crit Chance RED, YELLOW, QUINT. I then take Attack Speed BLUE because it is more efficient than crit chance blue which is awful.
When jungling (which I almost always do), I substitute 2 2% XP quints for 2 of my crit ones. This way I can take 24/6/0 and ditch utility altogether.