DTKs are derived rather than random so that the device doesn't have to store its own keys or identifiers, saving both space and flash wear. It can start with the initial seed and re-derive all the rest as needed.
A guid is 16 bytes per key * 6 keys per hour 24 hours per day 365 days makes less than a megabyte. Even accounting for additional data stored where guid is the key it feels not a big concern.
I imagine data stored like: Key: guid Contacts made: <timestamp, guid>[]