A "smart" phone lets you use software with better protocols than the government approved (designed) cell network. For example, turn off your cell radio but still using a messaging app (eg Matrix/XMPP) over open wifi (rotating macaddrs of course), instead of having to connect to cell towers to receive text messages.

I can't tell you which results in better opsec today for say carrying out government-disapproved commerce. But I can tell you that the privacy benefits of having a bona fide computer that you control in your pocket will continue to grow, while the cell network is stuck being forever subservient to government/commercial surveillance.