Properly done it would include any accounts recently accessed by that IP, should someone be using a family member's billing details to minimize the effect of a ban. Yes, this would affect friends that lend accounts, but it's a strong incentive to refrain from actionable things, and to choose friends better.
As you said, James, entire guilds and sometimes accessed friends get banned too. Duping and overt sexual harassment (e.g. MDK) used to be dealt with severely. The two people banned due to my paging (let me just say one founded the Sonoma guild that my handle sarcastically chronicles) rightfully earned theirs for a particular racial slur, and each bitterly complained afterward about losing his multiple accounts. It did not affect their guilds or friends, though, or one's father's accounts (same house and thus the same IP).
