1) it helps you differentiate between the people who KNOW when your birthday is. and those who just love to go on facebook.

2) it helps to weed out who STALKS you on facebook(to some extent). coz the people who post HAPPY BIRTHDAY!!! on your wall are the ones who are 1), or they actually paid attention to your profile(for one reason or another).

3) it helps to eliminate excessive amounts of notifications.

=)

(on the downside, if you reallly love getting the manymanymany birthday wishes, you'll be thoroughly disappointed)