"of those who do, almost nobody uses it."
Possibly because it does it at the wrong time.
Mine has an option to display a message when it starts -- so I could put my name and maybe even my landline number, in case its found by somebody honest (a few of which genuinely DO exist) who would be happy to tell me they had my phone.
It also has an option to require a PIN so that if somebody finds it, they can't make lots of calls to exhaust the credit.
Unfortunately, it doesn't display my message until AFTER the pin is displayed. So if I use both options, they won't see the message that would tell them whose phone it is.
Anyway, to perform its intended purpose, the PIN needs to be asked before making a call, and at the moment it only does that if the phone was switched off before being lost. So, I've got to know in advance that I intend to lose my phone, and remember to switch it off before doing so.
All in all, some good ideas but badly thought through such that they don't really achieve what was intended, let alone what is actually needed.