[blocke]

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.

[MJR]

What is "semi-private" data and if I do not give out my mobile phone number, how can anyone obtain it unless I call them?

[blocke]

Besides leaked databases and PI tools this is one I learned of in the past month:

There are various reverse caller databases floating around. I forget the name of the database but there was a talk at HOPE 2010 less than a month ago about abusing VOIP providers and caller ID spoofing to scan through and map large blocks of cellphone numbers at a time. Apparently there are wholesale resellers of caller ID lookup databases and some cellphone companies are more than happy to associate the name on the account with individual cellphone entries in their reverse databases.

With such a system if you know what possible area codes/exchanges they might be in and what their name is you had very good odds of finding AT&T and T-Mobile customers. Verizon wasn't really discussed.

Of course social engineering the number from you might be a lot easier the first time... ;)