User blog comment:Deshiba/Are you Critting me !?/@comment-5877442-20130520174741

First of all, Sion's passive and Phage slow procs are still based on chance (doesn't matter though).

In real battle, you're not going to strike their weak spot every time, so they may be making it more realistic. If you hit a crit, you can dramatically change the result of a battle; this can make for good plays and add a risk element to the game.

Finally, if everything's based on a certain chance, it's good to have a chance mechanism to make the game more interesting and unique.