I don't want to recycle old arguments but would just like to make a couple of points.
I agree with JimmyEngland that a helmet would probably provide some protection in almost any head impact. However, cycle helmets are designed to protect against hitting flat surfaces, particularly the ground, not against objects like posts. My guess is that hitting a post at 15mph might brake the helmet in half (it is only polystyrene) and your head would hit the post anyway.
I see your point but I think cycle helmets are designed to protect your head, regardless of surfaces. If the helmet were to break in half on impact with a post then the helmet would have taken most of the impact and hopefully you would end up with no more than a bump on your head.
At the end of the day it is down to personal choice at present and I don't doubt that some time in the future it will become law to wear one like with the motor cycle.
I wear one due to personal choice as I have a tricky road to ride on my way too and from work which results in cars, lorry's etc having no respect for a cyclist
Regards
Mandy