using only one id, i cannot speak for those who use multiple ids. but i could guess based on human nature: a person may want to accumulate goodwills on one of his ids and use the other ids as attack dogs or "echoing bugs." it would be a small penalty to block one's attacking dog or echoing bug ids, and it would be a big penalty to block his main id, which the person might wish to use to show off his talents. blocking all of one's ids would be like using the nuclear bomb.

so the webmaster can fine-tune the penalty by blocking some but not all of one's ids.