Pretty straight forward:why would you want to use one Crit-Rune in your Runepage(giving <1% Crit Chance)?
I have seen this from AD Carry players where it made atleast some sense,but there are also Supports (like Green Tea in this match) using this and I can not really imagine why less than 1% chance to land a critical strike would be better than have a bit more sustained damage.
You do not really benefit from it early game(in contrary,it might even deny you last hits when you get a critical hit while pushing as AD Carry) and as a Support player,you do not get any item with critical hit chance on it and relying on <1% chance does not seem to be worth that rune slot to me.Yet they use these runes and given that players like Green Tea are playing in great tournaments,they want to optimize everything they can so there has to be a point I miss out.
Best Answer
It's basically just for the luck factor. With one crit rune you won't give up too much sustained damage, but you'll still have the ability to crit.
Now this possibility (although it's very unlikely to happen soon) can be really game-changing. A Random Crit can make the difference between getting or missing a kill, and a single kill can change the outcome of a Lane completely.
Also this can confuse the enemy. It's completely unpredictable whether someone will crit or not and because of the low crit chance, most people assume that he won't crit.
So basically: It's simply a "luck" while not giving up too much sustained damage.