With the Crit Dmg Rune build, my main goal is to outlane and outfarm my opponent early on by rushing Cloak of Agility while he goes for B.F. Obviously, his is gonna' take a lot longer than mine and I can reap the benefits of having a Cloak of Agility for the Critical Chance AND Crit Dmg Runes for the additional damage. Note: for gangplank crit dmg runes are more viable since his Q has some base damage and will thus easier get more dmg when having crit dmg runes conclusion: AD per lvl is more useful as critical dmg runes since it doesn't require you to have any crit chance to gain the bonus effects. AD per lvl doesn't rely on any items, crit dmg does. Edit: Just wanted to add- you still really really shouldn't get Crit Damage runes anyway though, I just wanted to demonstrate that they are far superior in late game scaling. Getting Crit Damage runes screws over your early- and mid-game and that's essentially why no one does it. I can totally agree on Alacrity over one of the%-Bonus Dmg Runes, 10% only on targets that are below 40% HP is indeed very situational and getting +20 dmg on a 200 dmg aa is not all that much. Alacrity is constantly stacking throughout the game. This doesn't explain entirely why pros use one crit rune, but it does explain why the one crit rune has more worth than most people think. As far as I know, Crit chance in LoL doesn't follow the conventional way people think about% chance.

tbh, i think critical dmg runes need a buff.
you need to get over 350 attack damage without anyone damage runes to really see the difference.
the point is crit dmg can be seen as AD per lvl since it works best with a lot of AD.
maximum damage increase with crit dmg runes:
full marks, seals and quints(total 40% crit dmg) give a damage increase of: 15.3% as soon as you have bought your infinity edge.
normal crit dmg: 200% +
lethality 5-10%(5% for ranged, mastery) +
50% from infinity edge.
total: 255-260%
40/255 = 15.7% or 40/260 = 15.4% damage increase on crits (note that the reinforced armor mastery can take away a huge part of this since it is applied after these calculations and thus reduce this amount by about 12%, but this doesnt matter since this is applied to all crits, even the onces without crit dmg runes)
crit dmg cap for ranged champions: 300% (no other passives that increase that on ranged champions)
increased dmg: 17.6%(45/255) more then normal (but you have lost any AD armor or mr from runes so you will see later on that to succesfully use these crit dmg runes you need 400 AD at the least, which simply makes these runes not viable)
crit dmg cap for melee champions: 305% + passive increase (tryndamere)
increased dmg: 17.3%(45/260) more then normal, with tryndamere's passive tho the amount of dmg that is increased on crits gets even lower and thus less usefull.
alright so how does my damage change?
lets say i have 300AD since i want huge crits and i probably need a PD aswell so 300AD is about the max.
300 + 22(full flat AD), 18(flat AD + armor) or 52(full AD per lvl) = 322, 318 or 352 AD
322 * 2.55(ranged) = 821.1 dmg
318 * 2.55(ranged) = 810.9 dmg <- i would recommend to take this since your attacks will be much more deadly in early game, thus allowing you to farm or kill easier.
352 * 2.55(ranged) = 897.6 dmg
300 * 3.00(ranged full crit dmg) = 900 dmg <- i wouldn't recommend this, it just sucks as i show right here below, the glyph's are really inefficient when they are used for crit dmg, the seals are better then AD if you get to 300+ AD.
306.6 * 2.95(when you exchange crit dmg glyphs for AD per lvl) = 904.47
so what we see here is that the total damage increase is about 2 dmg compared to scaling AD and about 78 damage compared to full flat AD, but we still need to take in account that the critchance we have is probably not above 55% (PD + IE) so we are still sacrificing about 45% of that extra damage, making the 78 damage an increase of 43 dmg.
i do not know when the reinforced armor mastery(10% reduced crit dmg) takes place, if it reduces total dmg or the critical dmg the opponent has.
in case 1 you have to further reduce the 43dmg, since 10% of 900 is more then 10% of 810.
note: for gangplank crit dmg runes are more viable since his Q has some base damage and will thus easier get more dmg when having crit dmg runes
conclusion: AD per lvl is more useful as critical dmg runes since it doesn't require you to have any crit chance to gain the bonus effects. AD per lvl doesn't rely on any items, crit dmg does.
if you have seen any flaws in my calculations i would be glad to fix those. any missing parts could be added as well of course.
greetings,
Quimoth

Why Does No One Use Crit Dmg Runes 1

Some more notes on this:
The lower the target's armor is, the more effective armor reduction and penetration are (until you have penetrated to zero). The more armor penetration/reduction you get, the more effective each point of armor penetration you have is. This means, while Crit Damage may be better than Armor Penetration at 200 armor WITH NO OTHER PENETRATION/REDUCTION, if you add in something like Last Whisper (which is pretty much necessary on any physical attacker, especially if they have 200 armor), you increase the required armor to make Crit damage better to something like 330.
The other thing worth noting is that Armor Penetration only has an effect up until it reduces their armor to 0. As most champions start with something like 14-18 armor, this means that Crit Damage might be more effective at level 1. However, if you do the actual math for Shaco:
50 base damage + 20 from potion, 250% crit damage on deceive, (283% with crit damage runes), more armor penetration than they have armor (call it 14 armor), you get 175 net damage with Armor Pen and 173 with Crit Damage.
In short, stop ever suggesting crit damage on anyone.
Also, mad props to EnderA for posting this thread.