Interesting letter to sfist,  which typically loves the new rideshare companies like Lyft and Sidecar. The writer, apparently a cab driver, makes clear why these unlicensed cabs are a problem:
Taxi drivers are professional drivers with hundreds of thousands of city miles under their belts, intensive knowledge of city streets and each vehicle is inspected twice a year by the government of San Francisco and the SFO Airport Authority. These inspections insure that the three GPS tracking units, full motion video cameras, radios and other safety equipment is functioning and that the car is in compliance with DOT rules and regulations.
Yes, some cabbies are assholes, and yes, some cabbies don't know their way to certain places in the city, and of course some cabbies drive like crazed maniacs. Sadly though the more Lyft and Sidecar operate and undercut the legitimate transportation services the more often this will happen. How would you feel if you had spent countless dollars and hours getting to do your job for crappy pay and to be treated like shit, only to have someone come in that didn't do what you had to to get your job and do essentially the same thing for less money... making it so you couldn't feed your kids, or pay your rent
Lyft and Sidecar are very dangerous for the city and the traveling public by putting un-licensed, uninsured, untrained, amateur drivers on the streets.
TIme to call these what they are -- taxis -- and make them get permits like every other taxi in the city.