[xmm]

There is a lot of unnecessary information in your hypothetical message. What, beyond "[name] plans to rendezvous with [family member's name] at [rendezvous point] (about [time])" needs to be conveyed? Suddenly it's a very different message.

[kragen]

Well, if someone is listening to the girl's voice on the radio, they can guess her age. If her voice isn't on the radio, you as a family member have no reason to believe that it's really her. You need to plan out the route to the rendezvous point so that you know where to look for her if she doesn't make it, and she knows where she should go if you don't show up. If you (the family member) are trying to find a place to stay, or a way to buy gas, it may be crucial information to you that she has money, because that information may keep you from attempting trips into dangerous areas to get it; likewise, that she is unarmed, because, if you feel that you need to be armed, you know that her arrival will not provide that, and you still need to keep considering alternative options, like dropping half a grand at the pawn shop before they sell out. When you are having the discussion with her and discussing alternative possible plans for meeting up, the fact that she is unarmed is relevant to choosing among the alternative plans. You probably also need to discuss with her what things to take along on the trip; maybe she hasn't thought about the need for N95 masks in the recently burned zone, but maybe she can get one from someone at the station.

So really this kind of rendezvous arrangement benefits enormously from being able to discuss such plans at some length, but it would be terrible if random strangers were listening in. Without cellphones, you have to discuss this stuff in advance in order to successfully meet up with people; you can't pop off an SMS to ask them where they are or let them know you're running late.