It is semi-private data. I hate to say this but phone numbers are not exactly private anymore and anyone with clue and motivation can dig it up for pennies. But a user is justified to feel negative about such data being transmitted.
I could imagine a real use for it in the way the developer mentioned if it ends up acting like the only unique identifier that changes between specific phones and carriers. You're likely to port your phone number with you and thus the identifier travels with you.
Edit: I honestly don't know what kind of unique identifiers Google provides so the number could still be inappropriate.
