*Disclaimer* - I am an ap mid main, I have no bias on this topic.
Written from my toilet.
I just ran the numbers in Excel. Scaling AD runes (all types, but especially glyphs and seals) are better than crit damage runes. This is even more-so the case if you buy infinity edge (which you certainly will if you're focused on crit...) The following assumption is made:
-Scaling AD runes scale approximately equally with crit damage runes during a game (you start with 0 crit, build up as you go).
The logic is this - the +50% damage multiplier increases the damage from the ad you gained from runes, but it does nothing to the damage you would gain from additional crit damage (they stack additively). Unfortunately, it seems that in no scenario are crit damage runes superior to scaling ad runes; at nominal 200% crit damage amp (not including runes), you will do about the same damage from a crit whether you have crit runes or ad runes. At 250% damage amp (with IE purchased), you will do MORE damage with ad runes than with crit damage runes. Therefore, crit damage runes are obsolete. I was looking into making a full crit cheese rune page for ARAM. I figured that since crit is risky, there would be some potential reward from crit damage runes, but apparently not.
I know that crit is hot topic with rng and all that. This thread is neutral on that.
After evaluating Ashe's numbers, the following is concluded:
Ashe deals more damage with scaling ad runes than crit damage runes for all rune types, at all crit %, with or without IE, at all AD levels, with the following exceptions:
The damage equalizes at higher AD levels (approx. 300 AD), and then is VERY SLIGHTLY higher for crit runes after that. It's very minuscule, and not necessarily worth it. Overall, the two rune types seem to be more evenly matched for Ashe.