Jeff Kaplan has stated that smurfing is ok, as long as you are not throwing, greifing, etc, because you will get to your level very quickly, but account sharing is not, especially at a huge skill level difference. You either threw, bought an account, or sharing an account and are boosting, all of which are bannable. There is no excuse, and "playing with friends" is not acceptable. You are making a negative playing experience for other people, whether yours or the other.
Blizzard is going to be releasing stricter punishments soon for these things. I suggest everyone not account share, and I am suggesting that to both the ones using the accounts, and the ones giving them away.