Why do people sometimes have only one crit rune?
I was watching a youtube video about MadLife's thresh support to get some idea of how to improve my accuracy with his hooks. When the rune page came up, I saw only one crit rune in it. Mathematically with his .25% (1% since its rounded up) crit rune he will get 1 crit out of 100 auto attacks. Since this guy is a pro he wouldn't add it to his rune page unless there was some meaning behind it. So I'm here to ask, why have one crit rune in the first place? I read online that crit chance scales for each auto attack until u crit, then it resets back to your current crit chance; but even so does this really make a difference?
That, and I occasionally get to screw over my lane opponent with a lucky crit, or perhaps get a minion kill that I otherwise wouldn't have done quite enough damage to finish off.