I can see the aluminum construction of the Model S being a liability for adoption as a Taxi. The Crown Vic wasn't just the defacto Taxi for its size and torque, but because its body on frame construction was easy to repair. All the body panels could just be swapped out as needed, and some taxi companies and police departments even own their own frame straighteners specifically designed for the Crown Vic. In places like Manhattan the ease of repair made up for the addition running cost from fuel of such an antiquated and heavy design.
Although the Crown Vic is no longer an options for new fleet vehicle purchases, fact remains though that for some fleets like taxis in crowded cities, body repair will be a major factor in maintenance cost. Ease and cost of replacing panels on the Model S it a complete question mark at this point, and any company attempting to do repairs in house is going to need new equipment and training to work with aluminum. TIG welding instead of MIG or stick requires completely different equipment, a completely different skill set for welding, and even dent repair on aluminum is different from on steel.
I still think the Model S is a great fleet vehicle for corporate cars or taxis in less dense and wild traffic, but it'll probably be a long time before you see a aluminum bodied of any kind car painted yellow in Manhattan.