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.
You phone already stores lots of data every 10minutes, I am not sure if flash wear is a problem. Same with storage space.
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>[]
