While it is true no info is stored on the MB, that's not the reason they could not be used for ID and other information.
The MBs, when scanned, can yield all kinds of info (names, resort reservations, dates, phone numbers, home addresses, etc.) as all of that info is stored in a guest's MyDisneyExperience account (and other internal Disney files linkable to that.)
The access to some or all of that can be restricted from more basic MB scanners and limited only to all except higher echelons of CMs (security, for instance.)
But, for whatever internal reason, Disney (so far) has chosen not to use the MBs as "kid locators."