/nods

I agree with Steph. While Blizzard doesn't comment when people are banned, the general consensus is that it happens to people that are having a seriously negative influence on the game, exploiting a known bug for gain, or similar. Why this can't be 100% proven, it seems like common sense.

When multi-boxing you're in 100% control of your toons, you are staying within the game mechanics, and you're also paying for all of the accounts that you're using. Can you say absolutely that someone won't get banned for using Synergy or Multibox? Nope. But it's too much fun playing with 2 or more toons at a time for me to go back