Modified Live Viruses do confer a much longer duration of immunity than killed vaccines and do cause far fewer adverse reactions; however, they can and do revert to virulence in some cases. The thought of anyone's dog contracting rabies (no matter how rare the incidents may be) from an MLV rabies vaccine, resulting in all family members and friends having to undergo prophylactic rabies treatments themselves is daunting.
Personally, I will give my dogs the killed rabies vaccine even if the USDA licenses an MLV rabies vaccine in the future.
A couple of years ago, a batch of "non-inactivated" human rabies vaccines had to be recalled.
Recall of Rabies Vaccine (Human Diploid Cell), Imovax Rabies, Aventis Pasteur -- that was a healthy reminder that mistakes happen even with rabies vaccines.