Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
ok, you could be a bit confused, if you are asking why riven should be played, i answer you with 6 words : every midlaner is weak to cc.
The only midlaner who can counter you is ezreal, but with a nice stun and a block of his skillshots with your shield, you can easily kill him too.
Seriously, i am not trolling, a 500 hp on e, casted every 3 seconds, is op, i can't imagine that none though this except me.