Riven needs a particle for when she still has Wind Slash

This helps communicate vital information to the enemy in case they can't keep track of whether or not she used Wind Slash or not during her ultimate.

For those of you who would say it's up to the player to keep track of that, you can't realistically always know if Riven has used it. Sometimes your attention must be elsewhere (like peeling for Ashe), and you didn't see if she used Wind Slash.

It's the same case as with Champions like Aatrox, Kha'Zix, etc. Your enemies should know if Riven can execute them.

Besides, it gives Riot an excuse to add a cool new particle to Riven. ;)