The game is really played much quicker than one would think.  Certain groups of people spend all their time running around flanks going after archers.  Often times you don't even see someone coming before they've killed you from behind even though you're at the back of the lines.  Someone slinging might not see someone behind them and cast a missle, hitting a would-be assassin in the process.

Wasn't terribly clear, was I?  What I meant was you'd be casting forward, and with the follow through it might hit someone standing nearby.  Or casting in an overhand style or hilocopter style might cause you to hit someone while it is swinging behind you.  By "hit" I mean strike with the stone still in the pouch, or whipped with the cord.  Not hit with the projectile.