I am a former firefighter/paramedic. I think it'd be impossible to use helicopters for roof rescues in that situation. You had a broadcast mast on the one tower and commercial 2-way antennas on the other one... air turbulance due to the heat and smoke so thick you couldn't see where you were going. Did you see any rescue choppers around those towers in the news footage?
Yes - in fact the police on the helicopters were distressed NOT to see anybody on the roof to help and rescue.
BTW - around 1/4 of the rood of the North Tower was essentially free of smoke.
One day, after all the hurt somewhat dissipates, a book will be written of the failures at the WTC.