The disruption is that it surfaces all those problems which already exist for taxi drivers in the current system. They often work insane shifts, lose money if it's a slow day, etc. They have to enter into long exploitive contracts just to rent a medallion from the politically-connected rent-seeking monopolist who owns it.

Also, I often see drivers who don't resemble the photo ID in the cab at all. Likely they're unlicensed friends of the medallion renter, who help cover the insane shifts they rent the medallion for. But it negates much or all of the supposed value of taxis.

By giving those drivers an identical (somewhat crappy, as you point out) choice which doesn't lock them in, it's shaking up the taxi industry. And by reducing the value of the medallion it's hurting the monopolists who lobby to keep the number of medallions low, etc, which helps us in the long run with healthier markets.

As for Uber specifically, their value is surge pricing. And I say this as a customer, not a shareholder. I routinely wait for taxis that never come when I'm in cities that have banned Uber. But I never wait for an Uber where it exists - I can choose to pay if I want it. If I'm late for a meeting, I'll pay. It's a service that was unavailable before at any price.