Sometime one knows someone's birthdays with day and month, but no year. Sensitivity about age and all that.
In fact, in our CiviCRM 2.0 DB we have a few people with just the month/day information (presumably from import), but we are unable to edit those records without adding a year to the date, which we don't have, or removing the birthday all-together. While it's best to have someone's age, I'd rather be able to just recognize a birthday than nothing at all.
I posted on the suggestion board, asked to post an issue.