Cabs are a vital part of NYC transportation. Too many cabs will clog up streets causing traffic.
There are other reasons to regulate. To prevent predatory pricing. Without set prices cab drivers would start ripping people off. You can see the trouble with this with Uber's surge pricing. I needed a cab home from the airport but Uber wanted 4X pricing. Luckly cabs still exist, but in 5 years they might not. My 80 buck fare would be 320 bucks.
A lot of pro uber folks want to pretend that taxi regulations are purely protectionist. But it's really just quid pro quo. You can't tell cab drivers what to charge and make them drive people to anywhere in the city. They might not be profitable. Limiting supply is consultation price.
Well now that deal is done since cities allow Uber to compete with them unregulated.
I think the only real solution is to deregulate cabs. But a lot of people are going to be pissed when their cabbie informs them of surge pricing or that he doesn't deliver to certain areas.