RITO: Why do we have to pay to get 6 and 7 mastery?
Frankly, this is one of the most annoying things about the game right now. This is something Riot does to get people to spend IP or RP on champ tokens so they will be more likely to use RP for champs etc.... So unless you have all champs unlocked on your account(which i actually do...), you're passing up on unlocking a champ or expanding your rune page options -- things that give you the flexibility to get advantages in games.
So what does a 6 or 7 emblem really mean? Not much in terms of skill, because almost anyone can play a champ enough times to get the S's. Really it shows that you valued an emblem over expanding your play options or you already own everything you feel you need in terms of runes/champs. I'm not at all against riot doing stuff to make money, but this is "champ mastery", it should reflect ones skill with a particular champ rather than what one values most to spend IP or RP on.