I like how a 9 year old thread can get resurrected and finds a new audience with insightful comments.
I've always held the belief that the birthday ban came about in the Rutherford years when 1) he wanted to jettison anything Russell liked (the Daily Manna book back in the day had a place to mark down birthdays) and 2) a way to put a division between the Russelite-Bible Students (the 'evil slave' of the time period) and the newly named JWs, especially when there were family members on both sides. Eliminating family holidays like Thanksgiving and Christmas, as well as birthdays, drove a wedge between the families.