But from a UX standpoint they are vastly superior to having to deal with people trying to reach me on my US number while I'm in China, or trying to reach one cell phone when I'm carrying another.
Also, the telco operators love to nickel-and-dime consumers for things like roaming charges. I'd love to see more competitors come out and overthrow those rascals by doing everything over IP, creating worldwide MVNOs and slashing all roaming charges to essentially zero. Google Fi has done a nice job. Granted they exist to promote Android, but when I tell friends I have LTE data in 100+ countries without roaming charges, international voice calls for pennies directly from my cell phone, and viewing voice charges in real-time, they are all astounded. Unfortunately yes they too are a walled garden, but the user experience is amazing.