There's something to be said for choosing not to ban, like say forcing someone to pick between banning amumu or shen or what have you.
Not picking a champion though.. If you want to random, pick random (which has been conveniently moved to the top of the champion list!)
What? Explain further. What I think you mean is this:
Team A bans Blitz
Team B bans Malph
Team A bans Ez
Team B bans Taric
Team A skips ban
Team B bans Amumu
Team A first picks Shen.
Is that what your mean? Because if it is, you know, team A can accomplish that very same goal, while still banning someone not named shen or amumu. There are like 100 other champs still left that they could have banned.
Unless you meant another situation that I'm not familiar with, I can't see a reason to skip a ban. I can't recall a tourny game where a ban has been skipped.