Originally Posted by Doppy
Whenever people propose new legislation or regulations, I like to look at it the other way - what's the benefit.
As it appears that bad guys could simply lie about their DOB without too much trouble, requiring it seems to do nothing to improve security. Since I see no benefit, I don't have to get to your question of "what's the harm?" (Though I have already answered that question in detail in a related thread in Travel Saftey / Security.)
One potential benefit may be fewer false positives on the no-fly list. I suspect anyone who shares a name with someone on the list would consider this to be a great benefit.