Matt's further comments underscored how a 301 is useful for migrating to another domain (what other tool do you have, anyway?).
He also mentioned migrating within a site, for example if you change your CMS. Now there's a case where I prefer not to use a 301, but to retain the old URLs if at all possible. But sometimes the old URL scheme is so awful that it's important to change things. Always an informed trade off, isn't it.