There is also the convenience factor. I do remove my battery frequently from my phone but it is too easy to just leave it on. People will generally not want to hassle with it if they don't have to.If security is easier and requires less effort then it lowers the cost and will be favored more often and it will protect more people.

There are also many times that people have been unhappy because I have my phone off and unable to reach me. There are also other times you want to be available.

Unfortunately, if your phone is owned then they can just exfiltrate the data when normal data transfer occurs. The data will still need to be stored for transmission later though.

With the iphone, you also need to monitor Bluetooth and WiFi in addition to cellular activity. Hopefully they will also monitor these.

Depends on your use-case. If you only want to stop communications during a specified period of time (e.g.) during a "secret" conversation, then this is useful. If you have any apps installed that might be polling the 'net for information (e.g. checking the weather), then this is useless in the general case of "the phone is on my desk next to me while I'm working."