1) dissatisfaction with team make-up
2) one member of the team is afk/disconnected and never picks a toon
3) a friend signs on at the last minute and you'd rather team with a buddy.
2) Wrong. If someone is true AFK, and they don't touch the application at all during champ select, it puts you all back in the queue. You're dodging people who are in game, but refuse to pick a champion ("hard random"). Then it's back to 1) above.
3) If this happens during the 1.5 minutes window that you are *actually* in champ select, tough.